cancers Article Pharmacological Modulation of SAMHD1 Activity by CDK4/6 Inhibitors Improves Anticancer Therapy Marc Castellví 1,2,† , Eudald Felip 2,3,† , Ifeanyi Jude Ezeonwumelu 1,2 , Roger Badia 1,2 , Edurne Garcia-Vidal 1,2 , Maria Pujantell 1,2 , Lucía Gutiérrez-Chamorro 1,2 , Iris Teruel 2,3 , Anna Martínez-Cardús 2,3 , Bonaventura Clotet 1,2 , Eva Riveira-Muñoz 1,2 , Mireia Margelí 2,3 and Ester Ballana 1,2, * 1 IrsiCaixa AIDS Research Institute, Badalona, 08916 Catalonia, Spain; mcnadal89@gmail.com (M.C.); iezeonwumelu@irsicaixa.es (I.J.E.); rbadia@irsicaixa.es (R.B.); egvidal@irsicaixa.es (E.G.-V.); mpujantell@irsicaixa.es (M.P.); lgutierrez@irsicaixa.es (L.G.-C.); bclotet@irsicaixa.es (B.C.); eriveira@irsicaixa.es (E.R.-M.) 2 Health Research Institute Germans Trias i Pujol (IGTP), Hospital Germans Trias i Pujol, Universitat Autònoma de Barcelona, 08916 Badalona, Spain; efelip@irsicaixa.es (E.F.); iteruel@iconcologia.net (I.T.); amartinezc@igtp.cat (A.M.-C.); mmargeli@iconcologia.net (M.M.) 3 Institut Català d’Oncologia, Badalona, 08916 Catalonia, Spain * Correspondence: eballana@irsicaixa.es † Theses authors contributed equally to this work.  Received: 13 February 2020; Accepted: 16 March 2020; Published: 18 March 2020  Abstract: Sterile alpha motif and histidine-aspartic acid domain-containing protein 1 (SAMHD1) is a dNTP triphosphohydrolase involved in the regulation of the intracellular dNTP pool, linked to viral restriction, cancer development and autoimmune disorders. SAMHD1 function is regulated by phosphorylation through a mechanism controlled by cyclin-dependent kinases and tightly linked to cell cycle progression. Recently, SAMHD1 has been shown to decrease the efficacy of nucleotide analogs used as chemotherapeutic drugs. Here, we demonstrate that SAMHD1 can enhance or decrease the efficacy of various classes of anticancer drug, including nucleotide analogues, but also anti-folate drugs and CDK inhibitors. Importantly, we show that selective CDK4/6 inhibitors are pharmacological activators of SAMHD1 that act by inhibiting its inactivation by phosphorylation. Combinations of a CDK4/6 inhibitor with nucleoside or folate antimetabolites potently enhanced drug efficacy, resulting in highly synergic drug combinations (CI < 0.04). Mechanistic analyses reveal that cell cycle-controlled modulation of SAMHD1 function is the central process explaining changes in anticancer drug efficacy, therefore providing functional proof of the potential of CDK4/6 inhibitors as a new class of adjuvants to boost chemotherapeutic regimens. The evaluation of SAMHD1 expression in cancer tissues allowed for the identification of cancer types that would benefit from the pharmacological modulation of SAMHD1 function. In conclusion, these results indicate that the modulation of SAMHD1 function may represent a promising strategy for the improvement of current antimetabolite-based treatments. Keywords: SAMHD1; antimetabolite; CDK4/6; pemetrexed; HIV; cancer 1. Introduction Sterile alpha motif and histidine-aspartic acid domain-containing protein 1 (SAMHD1) is a deoxynucleotide triphosphate hydrolase [1]. Through its dNTPase activity, SAMHD1 maintains the intracellular dNTP pool at levels adequate for DNA replication and repair but below a potentially mutagenic threshold [2]. Mutations in SAMHD1 were first identified as being causative of Cancers 2020, 12, 713; doi:10.3390/cancers12030713 www.mdpi.com/journal/cancers Cancers 2020, 12, 713 2 of 19 Aicardi–Goutieres syndrome (AGS), a severe autoimmune disease [3], but later it was proposed as a tumor suppressor gene, since mutations in SAMHD1 are associated with different types of cancer [4,5] and its expression has been found to be inhibited in several types of tumors [6,7]. More recently, SAMHD1 has been implicated in the protection against cancer and chronic inflammation by limiting the release of single-stranded DNA [8]. SAMHD1 also acts as a viral restriction factor, limiting the permissiveness of cells to diverse viruses (reviewed in [9]), including human immunodeficiency virus (HIV-1) [10,11]. SAMHD1 inhibits retroviral replication at the reverse transcription (RT) step by maintaining the intracellular concentration of dNTP below the threshold required for reverse transcription of the viral RNA genome into DNA [10,11]. SAMHD1 activity is counteracted by the HIV-2 accessory protein Vpx, a tool that has been extensively used to study SAMHD1 function [10,12]. In addition, SAMHD1 function is inhibited by phosphorylation, a process controlled by cyclin-dependent kinases (CDK) and therefore tightly regulated during cell cycle progression [13–15]. Different pharmacological agents have been shown to block SAMHD1 phosphorylation, inducing SAMHD1 function and viral restriction [16–20]. Nucleoside analogues are commonly used in the treatment of viral infections and cancer [21]. Following phosphorylation by intracellular kinases, these analogues are structurally similarly to endogenous dNTP. SAMHD1 has been shown to affect the efficacy of nucleoside analogs, either used as antiretrovirals [22–26] or as chemotherapeutic drugs [27–29]. Active SAMHD1 catalyzes the hydrolysis and inactivation of a number of different nucleoside analogues [30,31], including cytarabine (Cytosar-U® ,Ara-C), a first line therapeutic agent for acute myelogenous leukaemia (AML) and SAMHD1 expression levels were negatively correlated with Ara-C treatment success in individuals with AML [27,29,32]. Here, we evaluated how SAMHD1 function modifies the efficacy of a wide range of nucleoside and non-nucleoside antimetabolites currently used to treat cancer. We show that SAMHD1 can either enhance or limit the efficacy of a number of chemotherapeutic drugs, allowing us to discern between those acting as enzyme substrates or competitors. Importantly, we identify anti-folate drugs as being affected by SAMHD1 expression and function. Moreover, pharmacological activation of SAMHD1 by highly selective CDK4/6 inhibitors significantly enhanced the efficacy of drugs acting as competitors. Thus, pharmacological modulation of SAMHD1 activity has the potential to improve antiviral and anti-cancer therapies and paves the way to the identification of malignancies that may be treated with new drug combinations. 2. Results 2.1. SAMHD1 Regulates Antiviral Efficacy of Antimetabolites in Primary Cells Primary monocyte-derived macrophages (MDMs) are susceptible to HIV-1 infection, and its replication capacity is dependent on SAMHD1 expression. Additionally, M-CSF-induced differentiation initiates MDM proliferation accompanied by SAMHD1 phosphorylation. Thus, HIV-1 infection of MDM provides an excellent model in which to test the activity of antimetabolite drug efficacy. Anti-HIV-1 activity of a panel of antimetabolite drugs used in cancer treatment was evaluated in MDMs in the presence or absence of SAMHD1, after transducing cells with HIV-2 Vpx (Figure 1A). As previously observed, Vpx-mediated degradation of SAMHD1 reduced the antiviral potency of the nucleoside reverse transcriptase inhibitor (NRTI) AZT compared to untreated macrophages, but did not change the activity of NVP, a non-nucleoside reverse transcriptase inhibitor (NNRTI) (Figure 1B). Conversely, degradation of SAMHD1 improved the anti-HIV-1 potency of AraC in MDM (Figure 1C). Cancers 2020, 12, 713 3 of 19 Cancers 2020, 12, x 3 of 19 Figure 1. (A) Degradation of sterile alpha motif and histidine-aspartic acid domain-containing protein Figure 1. (A) Degradation of sterile alpha motif and histidine-aspartic acid domain-containing protein 1 (SAMHD1) by HIV-2 Vpx enhances HIV-1 replication in monocyte-derived macrophages (MDMs). 1 (SAMHD1) by HIV-2 Vpx enhances HIV-1 replication in monocyte-derived macrophages (MDMs). MDMs previously treated or not with HIV-2 Vpx were infected with a VSV-pseudotyped HIV-1 GFP MDMs virus andpreviously treated replication or not2 with assessed days HIV-2 later by Vpx were infected measuring with a VSV-pseudotyped GFP expression. A 5-fold change HIV-1 GFP in HIV-1 virus and replication assessed 2 days later by measuring GFP expression. A replication was observed after Vpx-mediated SAMHD1 degradation. Mean ±SD of ten independent 5-fold change in HIV-1 replication was observed donors performed after Vpx-mediated in duplicate is shown. ASAMHD1 degradation. representative westernMean ±SD of ten blot showing independent degradation of donors performed in duplicate is shown. A representative western blot SAMHD1 expression in MDMs after Vpx treatment is also shown. (B) Decreased sensitivity of showing degradation of SAMHD1 AZT afterexpression Vpx-mediated in MDMs SAMHD1 after Vpx treatment degradation inisMDMs. also shown. Dose(B) Decreased response sensitivity of the NRTI AZT of AZT and after Vpx-mediated SAMHD1 degradation in MDMs. Dose response of the NNRTIs NVP, in wild-type () or SAMHD1-depleted (∆) MDMs. Inhibition of HIV infection was NRTI AZT and NNRTIs NVP, in wild-type measured (■) or SAMHD1-depleted as the percentage of GFP+ cells relative(Δ) MDMs. to the Inhibition of HIV infection no drug condition. Mean ±SD was of measured as at least ten the percentagedonors independent of GFP+ cells relative performed to the no is in duplicate drug condition. shown. Mean ±SD (C) SAMHD1 of at least modifies ten independent antiviral activity of donors AraC. Dose response of the AraC in wild-type () or SAMHD1-depleted (∆) MDM. Inhibition ofDose performed in duplicate is shown. (C) SAMHD1 modifies antiviral activity of AraC. HIV response infection of was the AraC in as measured wild-type (■) or SAMHD1-depleted the percentage of GFP+ cells relative (Δ)toMDM. the noInhibition of HIVMean drug condition. infection ±SD was of atmeasured least threeasindependent the percentage of GFP+ donors cells relative performed to the noisdrug in duplicate condition. shown. Mean ±SD Meanof±SD of at three at least least three independent independent donors performed experiments performed in in triplicate duplicateisisshown. shown.*Mean ±SD**of p < 0.05; p25 >10 - Pemetrexed Anti-folate 0.25 >25 >100 - Methotrexate Anti-folate 0.42 79.24 190 - EC50 ; Effective concentration required to block HIV-1 replication by 50%, FC; fold change or ratio of the EC50 without SAMHD1 and the EC50 with SAMHD1 (-/+), or inversely (+/-). 2.2. SAMHD1 Is Required for Antiviral Activity of CDK4/6 Inhibitors SAMHD1 is inactivated in proliferating cells by a mechanism that requires its phosphorylation [13–15]. SAMHD1 phosphorylation may be directly regulated by CDK1 or CDK2, whose activity is upstream controlled by CDK6 [14]. The antiviral activity of the highly selective CDK4/6 inhibitor palbociclib is dependent on SAMHD1 expression [14,19] (Figure 3A). Thus, the efficacy of two other specific CDK4/6 inhibitors, ribociclib and abemaciclib, was also evaluated in the presence or absence of SAMHD1. The three agents were tested at the concentration where palbociclib showed the highest efficacy in cell culture (1 µM, Figure 3A). As expected, the activity of all three CDK4/6 inhibitors was lost in the absence of SAMHD1, indicating that the efficacy of CDK4/6 inhibitors depends on SAMHD1 expression (Figure 3B). Interestingly, similar results were obtained when the multi-kinase inhibitor midostaurin was evaluated (Figure S1A), suggesting that activity of multiple types of kinase inhibitors may be influenced by SAMHD1 expression. To explore the cellular and molecular determinants of SAMHD1 requirement for kinase inhibitor function, SAMHD1 expression and phosphorylation was measured by Western blot. Both palbociclib and midostaurin blocked SAMHD1 phosphorylation, whereas SAMHD1 protein expression was not affected (Figure 3C, Figure S1B). In addition, we observed a concomitant dephosphorylation and decreased expression of Rb, a substrate of CDK6, suggesting that palbociclib and midostaurin also affect CDK6-mediated CDK2 phosphorylation of SAMHD1 (Figure 3C, Figure S1B). Cancers 2020, 12, 713 6 of 19 Cancers 2020, 12, x 6 of 19 Figure 3. Antiviral efficacy of CDK4/6 inhibitors depends on SAMHD1 expression. (A) Dose response Figure 3. Antiviral efficacy of CDK4/6 inhibitors depends on SAMHD1 expression. (A) Dose response of the CDK4/6 inhibitor Palbociclib, in wild-type () or SAMHD1-depleted (∆) MDM. Inhibition of HIV of the CDK4/6 inhibitor Palbociclib, in wild-type (■) or SAMHD1-depleted (Δ) MDM. Inhibition of infection was measured as the percentage of GFP+ cells relative to the no drug condition. Mean ±SD HIV infection was measured as the percentage of GFP+ cells relative to the no drug condition. Mean of at least ten independent donors performed in duplicate is shown. (B) CDK4/6 inhibitors lose ±SD of at least ten independent donors performed in duplicate is shown. (B) CDK4/6 inhibitors lose antiviral activity in SAMHD1-depleted macrophages. As in (A), dose response of two other CDK4/6 antiviral activity in SAMHD1-depleted macrophages. As in (A), dose response of two other CDK4/6 inhibitors, ribociclib (left panel) and abemaciclib (right panel), in wild-type () or SAMHD1-depleted inhibitors, ribociclib (left panel) and abemaciclib (right panel), in wild-type (■) or SAMHD1-depleted (∆) MDMs. Mean ±SD of two independent donors performed in duplicate is shown. (C) Palbociclib (Δ) MDMs. Mean ±SD of two independent donors performed in duplicate is shown. (C) Palbociclib blocks SAMHD1 inactivation by phosphorylation. Western blot analysis of lysates of untreated MDMs blocks SAMHD1 inactivation by phosphorylation. Western blot analysis of lysates of untreated (no drug, ND) or macrophages treated with palbociclib at the indicated doses. Membranes were blotted MDMs (no drug, ND) or macrophages treated with palbociclib at the indicated doses. Membranes with an anti phospho-SAMHD1 antibody, total SAMHD1, anti phosho-pRB and total pRB. Hsp90 were blotted with an anti phospho-SAMHD1 antibody, total SAMHD1, anti phosho-pRB and total antibody was used as control. A representative donor is shown. * p < 0.05; ** p < 0.005; *** p < 0.0005. pRB. Hsp90 antibody was used as control. A representative donor is shown. *p < 0.05; ** p < 0.005; *** p < 0.0005. Inhibition of CDK4/6 Enhances Antiviral Activity of Antimetabolites 2.3. Pharmacological CDK4/6 2.3. inhibitorsInhibition Pharmacological activate ofSAMHD1 functionAntiviral CDK4/6 Enhances throughActivity the inhibition of its phosphorylation. of Antimetabolites Thus, we evaluated the capacity of palbociclib to modify the activity of antimetabolites. Antiviral activity CDK4/6 inhibitors of pemetrexed and activate SAMHD1 fluorouracil function through were evaluated alone or the inhibition ofwith in combination its phosphorylation. palbociclib in Thus, we evaluated primary macrophages. the capacity of palbociclib to modify the activity of antimetabolites. Antiviral activity of pemetrexed Pemetrexed inhibitedandHIV-1fluorouracil replicationwere in aevaluated alone or manner, dose-dependent in combination although with palbociclib with limited in primary macrophages. potency (EC50 = 0.1 µM, Figure 4A, black line). Combination of pemetrexed with increasing Pemetrexed concentrations inhibited(EC of palbociclib HIV-1 replication in a dose-dependent manner, although with limited 50 = 0.12 µM) enhanced the antiviral potency of the antimetabolite potency (Figure 4A,B,(ECleft = 0.1 µM, 50 panels). TheFigure 4A, black calculation of the line). Combination combination of pemetrexed index (CI) indicated strong with synergy increasing concentrations of palbociclib (EC = 0.12 µM) enhanced the antiviral potency (CI ≤ 0.041 for palbociclib at 0.04 µM combined with different concentrations of pemetrexed, Table 2). 50 of the antimetabolite (Figure 4A,B, Interestingly, left panels). pemetrexed andThe calculation palbociclib of the activity, as combination index (CI)effect well as the synergistic indicated strong observed synergy in drug combinations, were lost in the absence of SAMHD1, (Figure 4A,B, right panels). Furthermore, the 2). (CI≤0.041 for palbociclib at 0.04 µM combined with different concentrations of pemetrexed, Table Interestingly, combination pemetrexed of pemetrexed andthe with palbociclib multi-kinaseactivity, as well inhibitor as the synergistic midostaurin (EC50 = 0.62effect µM)observed in drug also showed combinations, a highly synergistic were lost effect in the when absencewas SAMHD1 of SAMHD1, (Figure 4C,D, expressed (Figure 4A,B, left right panels). panels, Furthermore, Table 2), an effect the combination of pemetrexed with the multi-kinase inhibitor that was lost when SAMHD1 depleted cells (Figure 4C,D, right panels). midostaurin (EC 50 = 0.62 µM) also showed a highly synergistic effect when SAMHD1 was expressed (Figure 4C,D, left panels, Table 2), an effect that was lost when SAMHD1 depleted cells (Figure 4C,D, right panels). On the other hand, combination of the nucleoside analogue fluorouracil or the multikinase inhibitor midostaurin with palbociclib showed more limited effects, i.e., palbociclib partially Cancers 2020, 12, x 7 of 19 enhanced the antiviral potency of fluorouracil or midostaurin in the presence of SAMHD1 (Figure S2). Combination index calculation indicated synergy at specific concentrations, although CI were Cancers 100-fold 2020, 12, 713 lower compared to palbociclib-pemetrexed drug interactions (Table 2) and antagonic or 7 of 19 additive effects were also seen. Figure 4. Pharmacological activation of SAMHD1 enhances antiviral activity of antimetabolites. (A) 4. Pharmacological FigureRelative activation effect of the combination of SAMHD1 enhances of palbociclib-pemetrexed antiviral measured activity as antiviral of Inhibition activity. antimetabolites. (A) Relative of HIVeffect of the infection combination with of palbociclib-pemetrexed increasing doses measured of palbociclib and pemetrexed as antiviral was measured. activity. of Percentage Inhibition of HIVGFP+ cells relative infection to the no drug with increasing condition doses is shown inand of palbociclib presence (left panel)was pemetrexed or absence measured.(right Percentage panel) of of SAMHD1. Mean ±SD of at least three independent donors performed in duplicate is shown. GFP+ cells relative to the no drug condition is shown in presence (left panel) or absence (right panel) of (B) As SAMHD1. Mean ±SD of at least three independent donors performed in duplicate is shown. (B) As in (A), relative effect of pemetrexed alone (white bars) or in combination with a fixed dose of palbociclib 0.04 µM (black bars), in the presence (left panel) or absence (right panel) of SAMHD1. Mean ±SD of at least three independent donors performed in duplicate is shown. (C) Relative effect of the combination of midostaurin-pemetrexed measured as antiviral activity. Inhibition of HIV infection with increasing doses of midostaurin and pemetrexed was measured. Percentage of GFP+ cells relative to the no drug condition is shown in presence (left panel) or absence (right panel) of SAMHD1. Mean ±SD of at least three independent donors performed in duplicate is shown. (D) As in (C), relative effect of pemetrexed (PTX) alone (white bars) or in combination with a fixed dose of midostaurin 0.2 µM (black bars), in the presence (left panel) or absence (right panel) of SAMHD1. Mean ±SD of at least three independent donors performed in duplicate is shown. PD, palbociclib; PTX, pemetrexed; MID, midostaurin. * p < 0.05; ** p < 0.005; *** p < 0.0005. Cancers 2020, 12, 713 8 of 19 Table 2. Combination index values for pemetrexed and fluorouracil combinations with palbociclib and midostaurin. Macrophages were treated with the different at indicated concentrations in combination with 0.04 µM of palbociclib or 0.2 µM of midostaurin and infected with HIV-1 for 48 h. Drug efficacy was analyzed by measuring inhibition of HIV-1 replication by flow cytometry. Drug Combination Combination Index (CI) Effect Pemetrexed 25 0.0049 Synergy + 5 0.0097 Synergy Palbociclib 0.04 µM 1 0.0285 Synergy 0.2 0.0415 Synergy 0.04 0.0673 Synergy Pemetrexed 25 0.079 Synergy + 5 0.069 Synergy Midostaurin 0.2 µM 1 0.056 Synergy 0.2 0.045 Synergy 0.04 0.064 Synergy Fluorouracil 5 1.871 Antagonism + 1 0.572 Synergy Palbociclib 0.04 µM 0.2 0.658 Synergy 0.04 1.818 Antagonism 0.008 2.967 Antagonism Midostaurin 5 2.074 Antagonism + 1 0.427 Synergy Palbociclib 0.04 µM 0.2 0.223 Synergy 0.04 0.419 Synergy 0.008 0.324 Synergy CI values were calculated using the mean values of three different experiments. Values were calculated using CompuSyn software. CI < 1, synergy; CI > 1, antagonism; CI = 1, additive. On the other hand, combination of the nucleoside analogue fluorouracil or the multikinase inhibitor midostaurin with palbociclib showed more limited effects, i.e., palbociclib partially enhanced the antiviral potency of fluorouracil or midostaurin in the presence of SAMHD1 (Figure S2). Combination index calculation indicated synergy at specific concentrations, although CI were 100-fold lower compared to palbociclib-pemetrexed drug interactions (Table 2) and antagonic or additive effects were also seen. Overall, these results suggest that pharmacological activation of SAMHD1 can significantly enhance the efficacy of antimetabolites, through a mechanism that is dependent on SAMHD1 expression and regulation. 2.4. Cytotoxic Efficacy of Antimetabolites Is Enhanced by CDK4/6 Inhibitors Anticancer drugs are specifically designed to inhibit cell growth, thus we evaluated cytotoxic efficacy of the antimetabolites pemetrexed and fluorouracil in combination with the CDK4/6 inhibitor palbociblib in the TZM-bl cell line and in two distinct breast cancer cell lines, MDA-MB-468 and T47D (Figure S3). As expected, all drugs tested resulted in decreased cell metabolic activity in all cell lines, reflecting the number of viable cells under defined conditions. The calculation of 50% cytotoxic concentrations (CC50 ) showed significant differences between them (Table S1). The combination of pemetrexed with palbociclib enhanced the cytotoxicity of the antimetabolite in all cell lines tested (Figure 5A–C). Importantly, the calculation of the combination index indicated a synergistic effect in all cases, with the cytotoxic evaluation being comparable to the results obtained when antiviral efficacy was measured (Table 3). The combination of fluorouracil with palbociclib enhanced fluorouracil potency in TZM-bl and T47D cells but not in MDA-MB-468 cell line (Figure 5A–C, Table 3). Interestingly, in MDA-MB-468 cells, although the expression of SAMHD1 was similar to other lines, Rb and pRb were not detected, either at the mRNA or protein level, (Figure S3), demonstrating a synergistic effect in all cases, with the cytotoxic evaluation being comparable to the results obtained when antiviral efficacy was measured (Table 3). The combination of fluorouracil with palbociclib enhanced fluorouracil potency in TZM-bl and T47D cells but not in MDA-MB-468 cell line (Figure 5A–C,Cancers Table2020, 3).12, Interestingly, 713 in MDA-MB-468 cells, although the expression of SAMHD19was of 19 similar to other lines, Rb and pRb were not detected, either at the mRNA or protein level, (Figure S3), demonstrating the importance of cell cycle proteins which putatively may affect SAMHD1 function the importance of cell cycle proteins which putatively may affect SAMHD1 function in determining in determining palbociclib–antimetabolite palbociclib–antimetabolite drug drug combination combination efficacy. efficacy. FigureFigure 5. Pharmacological 5. Pharmacologicalactivation activation ofof SAMHD1 SAMHD1 enhances enhances cytotoxicity cytotoxicity of antimetabolites. of antimetabolites. (A–C) Effect (A–C) Effectononcell viability cell of palbociclib–pemetrexed viability of palbociclib–pemetrexedcombination in TZM-bl, T47D combination and MDA-MB-468 in TZM-bl, T47D andcell lines, MDA-MB-468 respectively. Left panels, cytotoxic activity of palbociclib alone (5 µM, white bars), pemetrexed cell lines, respectively. Left panels, cytotoxic activity of palbociclib alone (5 µM, white bars), alone (black bars at 0.2, 1 or 0.04 µM for TZM-bl, T47D and MDA-MB-468 respectively) or the combination pemetrexed alone (black bars at 0.2, 1 or 0.04 µM for TZM-bl, T47D and MDA-MB-468 respectively) of both drugs at the same concentration (grey bars). Right panels, cytotoxic activity of palbociclib alone (5 µM, white bars), fluorouracil alone (5 µM, black bars) or the combination of both drugs at the same concentration (grey bars). Drug concentrations were chosen depending calculated CC50 under specific experimental conditions (Table S1). * p < 0.05; ** p < 0.005; *** p < 0.0005. PD, palbociclib; PTX, pemetrexed; FU, 5-fluorouracil. Cancers 2020, 12, 713 10 of 19 Table 3. Combination index values for pemetrexed and fluorouracil combinations with palbociclib. TZM-bl, MDA-MB-468 and T47D cells were treated with indicated concentrations of pemetrexed or fluorouracil in combination with 5 µM of palbociclib. Cytotoxic effect of the drugs was tested by MTT assay. Cell Type Drug Combination Combination Index (CI) Effect Pemetrexed 1 0.806 Synergy + 0.2 0.689 Synergy Palbociclib 5 µM 0.04 0.71 Synergy 0.008 0.72 Synergy TZM-bl Fluorouracil 5 0.726 Synergy + 1 0.766 Synergy Palbociclib 5 µM 0.2 0.764 Synergy 0.04 0.735 Synergy Pemetrexed 5 0.783 Synergy + 1 0.707 Synergy 0.2 0.779 Synergy Palbociclib 5 µM 0.04 0.874 Synergy T47D Fluorouracil 25 0.848 Synergy + 5 0.804 Synergy Palbociclib 5 µM 1 0.966 Additive 0.2 1.01 Additive Pemetrexed 1 0.886 Synergy + 0.2 0.886 Synergy Palbociclib 5 µM 0.04 0.921 Synergy 0.008 1.01 Additive MDA-MB-468 Fluorouracil 25 2.844 Antagonism + 5 1.745 Antagonism Palbociclib 5 µM 1 1.356 Antagonism 0.2 1.125 Antagonism CI values were calculated using the mean values of three different experiments. Values were calculated using CompuSyn software. CI < 1, synergy; CI > 1, antagonism; CI = 1, additive. 2.5. Alternative Pathways of dNTP Metabolism Control Are Responsible for Drug Synergy To further explore the mechanism underlying the synergistic effect observed when combining antimetabolites with the CDK4/6 inhibitor palbociclib, protein expression in primary macrophages treated with pemetrexed, fluorouracil and palbociclib, alone or in combination was evaluated (Figure 6A). As expected, palbociclib alone inhibited phosphorylation of pRb and SAMHD1, therefore activating its dNTP triphosphohydrolase function and subsequently reducing the intracellular dNTP pool. Interestingly, pemetrexed and fluorouracil treatment resulted in different effects, i.e., while fluorouracil acts similarly to palbociclib, pemetrexed did not decrease the phosphorylation of pRb and SAMHD1. Although pemetrexed activity is dependent on SAMHD1, its mechanisms of action do not directly affect SAMHD1 phosphorylation, providing evidence for the stronger synergy observed in the pemetrexed–palbociclib drug combination compared to fluorouracil–palbociblib. Thus, antifolates such as pemetrexed inhibit the dNTP pool by a mechanism not directly affecting SAMHD1 phosphorylation and effectively synergize with palbociclib, which actually induces SAMHD1 activation. On the other hand, when two compounds directly affecting SAMHD1 phosphorylation (i.e., fluorouracil and palbociclib) are combined, the synergic effect is less potent. do not directly affect SAMHD1 phosphorylation, providing evidence for the stronger synergy observed in the pemetrexed–palbociclib drug combination compared to fluorouracil–palbociblib. Thus, antifolates such as pemetrexed inhibit the dNTP pool by a mechanism not directly affecting SAMHD1 phosphorylation and effectively synergize with palbociclib, which actually induces CancersSAMHD1 2020, 12, 713 activation. On the other hand, when two compounds directly affecting11SAMHD1 of 19 phosphorylation (i.e., fluorouracil and palbociclib) are combined, the synergic effect is less potent. Figure 6. Regulation of dNTP pool is responsible for drug synergy. (A) Protein expression in MDMs Figure 6. Regulation treated of dNTP with palbociclib (PD)pool at 1 is µM,responsible pemetrexed for(PTX) drugand synergy. (A) Protein fluorouracil expression (FU), both at 5µM inandMDMs treated with palbociclib the corresponding drug(PD) at 1 µM, pemetrexed combinations PD+PTX and(PTX) PD+FU.andHsp90 fluorouracil was used (FU), as aboth at 5µM loading and the control. corresponding A representativedrugblotcombinations is shown. (B)PD+PTX Proposedand PD+FU. model of drugHsp90 was used interactions. as a loading Antimetabolites control. A affecting representative dNTP synthesis blotsuch is shown. (B) Proposed as pemetrexed model inhibit dNTP of adrug pool by interactions. mechanism Antimetabolites not directly affecting SAMHD1 affecting dNTP activation and thus synthesis suchsynergy with anticancer as pemetrexed drugs inhibit affecting dNTP poolSAMHD1 phosphorylation by a mechanism not as palbociclib directly affecting is higher SAMHD1 comparedand activation to compounds thus synergyalsowith targeting SAMHD1 anticancer function drugs (i.e., fluorouracil) affecting or exclusively SAMHD1 phosphorylation as affecting SAMHD1 (i.e., midostaurin). As a consequence, the antiviral and cytotoxic efficacy of palbociclib is higher compared to compounds also targeting SAMHD1 function (i.e., fluorouracil) or antimetabolites is significantly enhanced when used in combination in vitro. exclusively affecting SAMHD1 (i.e., midostaurin). As a consequence, the antiviral and cytotoxic efficacy 2.6. SAMHD1 of antimetabolites Is Expressed inisDifferent significantly Tumorenhanced Tissues when used in combination in vitro. To further explore the potential value of modulating SAMHD1 function in cancer patients, 2.6. SAMHD1 is Expressed in Different Tumor Tissues we evaluated SAMHD1 expression in different tumor tissues by immunohistochemistry (IHC) in paraffin-embedded To further exploretissues. SAMHD1 the potential was value of clearly detected modulating in at least SAMHD1 two cancer function tissuepatients, in cancer types we susceptible evaluated to beingexpression SAMHD1 treated within antimetabolites—pancreatic different tumor tissuesadenocarcinoma and lung large (IHC) by immunohistochemistry cell in carcinoma (Figure 7A,B). In both cases, SAMHD1 was significantly expressed in a high percentage of paraffin-embedded tissues. SAMHD1 was clearly detected in at least two cancer tissue types malignant cells. susceptible to being treated with antimetabolites—pancreatic adenocarcinoma and lung large cell In addition, IHC data of 17 different types of human tumors including 202 different samples carcinoma (Figure 7A,B). In both cases, SAMHD1 was significantly expressed in a high percentage of from human protein atlas were also analyzed (www.proteinatlas.org, [33]). Although SAMHD1 was malignant expressedcells. in all types of tumors, the degree of expression was significantly variable, ranging from In addition, undetectable IHCtodata levels high of 17 different protein expressiontypes levels.ofOverall, human70% tumors including of all tumors 202 different expressed SAMHD1samples to from humanextent, a certain protein atlas its whereas were also analyzed expression could not (www.proteinatlas.org, be detected in 30% of cases [33]). Although (Figure SAMHD1 7C). These results was expressed in allthat demonstrate types of tumors, SAMHD1 the degree is expressed of expression in patient tumor sampleswasbut significantly also suggestvariable, ranging that modulation of from SAMHD1 function might be feasible at least in a subgroup of cancer types. Cancers 2020, 12, x 12 of 19 undetectable levels to high protein expression levels. Overall, 70% of all tumors expressed SAMHD1 to a certain extent, whereas its expression could not be detected in 30% of cases (Figure 7C). These results demonstrate that SAMHD1 is expressed in patient tumor samples but also suggest that Cancers 2020, 12, 713 12 of 19 modulation of SAMHD1 function might be feasible at least in a subgroup of cancer types. Figure7.7.Expression Figure ExpressionofofSAMHD1 SAMHD1protein proteininin tumor samples tumor from samples cancer from patients. cancer (A,B) patients. IHCIHC (A–B) staining of staining SAMHD1 in pancreas (A) and lung (B) tumor samples. Morphology of tumor cells is shown of SAMHD1 in pancreas (A) and lung (B) tumor samples. Morphology of tumor cells is shown by by routine hematoxylin stain of paraffin embedded tumor sections. SAMHD1 is stained in brown. Original routine hematoxylin stain of paraffin embedded tumor sections. SAMHD1 is stained in brown. magnification ×200. Scale bar, 20 µm. (C) Percentage of tumors expressing SAMHD1, depending on its Original magnification ×200. Scale bar, 20 µm. (C) Percentage of tumors expressing SAMHD1, relative expression in IHC as classified in Human Proteome Atlas (www.proteinatlas.org, IHC data of depending on its relative expression in IHC as classified in Human Proteome Atlas SAMHD1 expression from 17 different tumor types were retrieved and classified according to protein (www.proteinatlas.org, IHC data of SAMHD1 expression from 17 different tumor types were expression levels. ND, not detected. retrieved and classified according to protein expression levels. ND, not detected. 3. Discussion 3. Discussion Nucleotide metabolism plays a central role in cell proliferation, transformation and tumor Nucleotide progression metabolism [34]. Therefore, plays aofcentral inhibition rolesynthesis nucleotide in cell proliferation, transformation has been commonly used in theand tumor treatment progression [34]. Therefore, inhibition of nucleotide synthesis has been of cancer, infectious diseases and immune-mediated diseases [34]. By degrading cellular dNTPs, commonly used in the treatment of cancer, infectious diseases and immune-mediated diseases [34]. SAMHD1 plays an important role in the homeostatic balance of cellular dNTPs and, thus, it may By degrading cellular dNTPs, be SAMHD1 a modulator plays an of clinical important efficacy role in the homeostatic of nucleotide-based balance treatments. of cellular SAMHD1 has dNTPs and, thus, been shown to actit may be a modulator of clinical efficacy of nucleotide-based treatments. SAMHD1 as a resistance factor to nucleoside-based chemotherapies by hydrolyzing their active triphosphate has been shown to act as a thereby metabolite, resistance factor the reducing to response nucleoside-based chemotherapies to the anticancer by hydrolyzing their active drugs [28,30]. triphosphate metabolite, thereby reducing the response to the Here, we show that SAMHD1 can either enhance or limit efficacy of a wide anticancer drugs range[28,30]. of antimetabolites, Here, we show that SAMHD1 can either enhance or limit including nucleoside analogues but also anti-folate drugs. Interestingly, antifolate efficacy of a widedrugsrange suchof antimetabolites, as pemetrexed orincluding methotrexate nucleoside showed analogues increasedbut also anti-folate potency drugs. Interestingly, in SAMHD1-expressing cells, antifolate whereas nucleoside analogues efficacy was either limited or enhanced by SAMHD1. Attempts to predictcells, drugs such as pemetrexed or methotrexate showed increased potency in SAMHD1-expressing the whereas effect nucleosideonanalogues of SAMHD1 efficacy drug efficacy was based oneither limitedbase the specific or enhanced targeted orby chemical SAMHD1. Attempts structure to did predict not showtheanyeffect clearof SAMHD1 although correlation, on drug purine efficacynucleoside based on analogues the specificmaybase betargeted more proneor chemical to gain activity in SAMHD1-depleted cells. Based on previous data showing that beyond endogenous be structure did not show any clear correlation, although purine nucleoside analogues may more dNTPs, prone to gain activity triphosphorylated in SAMHD1-depleted nucleoside analogues can be cells. Based by hydrolyzed on SAMHD1 previous [28–30,35] data showing that beyond and considering endogenous dNTPs, triphosphorylated nucleoside analogues can be hydrolyzed that recombinant SAMHD1 exhibits Ara-CTPase activity in vitro [29], we hypothesized that compounds by SAMHD1 [28– whose activity is enhanced in the absence of SAMHD1 are enzyme substrates. On the contrary, and we 30,35] and considering that recombinant SAMHD1 exhibits Ara-CTPase activity in vitro [29], as hypothesized that compounds whose activity is enhanced in the absence of previously demonstrated for NRTI [23–25], compounds that lose activity in the absence of SAMHD1 SAMHD1 are enzyme would be competing with the intracellular dNTP pool, which is lower when SAMHD1 is active. Accordingly, anti-folates showed higher activity when SAMHD1 effectively limits the dNTP pool. Cancers 2020, 12, 713 13 of 19 As previously reported, our data indicate that SAMHD1 decreases activity of drugs used against AML (AraC and clofarabine) and other types of hematological cancers like nelarabine for T-lymphoblastic lymphoma, cladribine used in the treatment of hairy cell leukemia and B-cell chronic lymphocytic leukemia, and approved for the treatment of relapsing-remitting multiple sclerosis. On the contrary, SAMHD1 enhances the activity of drugs used to treat solid tumors, such as floxuridine, most often used in the treatment of colorectal cancer, fluorouracil, used for colon cancer, esophageal cancer, stomach cancer, pancreatic cancer, breast cancer, and cervical cancer, and anti-folate drugs, such as methotrexate, used against a number of cancers but also to treat autoimmune diseases, such as psoriasis, rheumatoid arthritis, and Crohn’s disease, or pemetrexed, used to treat non-small cell lung cancer [36]. Thus, the data presented here expand the number of anticancer drugs known to be affected by SAMHD1 function and the number of diseases that could benefit from the development of pharmacological inhibitors or activators of SAMHD1 function. It is known that SAMHD1 function is tightly linked to cell cycle control as its activity is regulated by CDK-dependent phosphorylation (reviewed in [9]). We and others have identified drugs that activate SAMHD1 function by impeding its phosphorylation [16–20]. Such drugs may be suitable for combination therapies including antimetabolites, especially when combined with anti-folate drugs, as synergistic effects were seen across all cell models tested. Detection of the phosphorylated form of SAMHD1 and expression of related cell cycle proteins allowed us to suggest a mode of action where anti-folate drugs would diminish the dNTP pool by affecting the synthesis of nucleotide precursors, whereas CDK inhibitors deplete dNTPs by activating SAMHD1 triphosphohydrolase activity [19] (Figure 6B). Although our data provide strong evidence of the key role of SAMHD1 in determining efficacy of CDK4/6 inhibitors, further work will have to determine the relative contribution of SAMHD1 in each type of tumor, by, for example, performing additional evaluations in SAMHD1 in vitro and/or in vivo knock-out models. From our data, we cannot rule out the possibility that synergy observed in breast cancer cell lines might be not entirely dependent on SAMHD1, but due to a combination of different factors that deserve further investigation. Antimetabolites were the first class of cytotoxic drugs systematically tested in clinical trials that elicited complete clinical responses as monotherapies, albeit with inevitable relapse [37]. Combination chemotherapy still constitutes the current paradigm to achieve systemic disease control in clinical oncology. Thus, our results might represent the first step of a novel treatment strategy directed to the activation of SAMHD1 function. In this sense, the choice of highly selective CDK4/6 inhibitors as the first clinically effective CDK4/6 inhibitors developed [38,39] is of special relevance. Currently, inhibition of CDK4/6 in combination with endocrine therapies is the treatment option in hormone receptor-positive/HER2-negative advanced breast cancer. CDK4/6 inhibitors offer an effective and tolerable treatment that can be combined with other therapies and thus harbors therapeutic potential for multiple cancers [40,41]. Based on our data, CDK4/6 inhibitors could boost antimetabolite-based anticancer therapies, especially for drugs whose activity is enhanced by SAMHD1, which highlights the need for a priori testing drug efficacy depending on SAMHD1 function. We have applied here a screening approach based in anti-HIV-1 activity in primary macrophages that either express SAMHD1 or do not. Our system has turned out to be a reliable and sensitive measure of SAMHD1 drug dependency for several reasons: (i) SAMHD1 expression is easily modulated, (ii) HIV-1 reverse transcription is a process highly sensitive to SAMHD1-mediated dNTP pool sizes changes and can be easily monitored, (iii) cell cycle initiation and progression is not deregulated and (iv) considers inter-individual differences by using primary cells from different donors. Indeed, cytotoxicity data obtained in different cell lines confirmed the results from the antiviral-based screening, except when expression of pRb was deregulated. RB is one of the key factors in cell cycle control that is phosphorylated by activation of CDK4 and CDK6 following cell cycle entry from GO to G1 [42], as is the case of SAMHD1. Cancers 2020, 12, 713 14 of 19 4. Materials and Methods 4.1. Cells Peripheral blood mononuclear cells (PBMC) were obtained from the blood of healthy donors using a Ficoll–Paque density gradient centrifugation and monocytes were purified using negative selection antibody cocktails (StemCell Technologies, Vancouver, BC, Canada) as described before [43]. Monocytes were cultured in complete culture medium: RPMI 1640 medium supplemented with 10% heat-inactivated fetal bovine serum (FBS; Thermo Fisher Scientific, Waltham, MA, USA), penicillin and streptomycin (Thermo Fisher Scientific, Waltham, MA, USA) and differentiated to monocyte derived macrophages (MDM) for 4 days in the presence of monocyte-colony stimulating factor (M-CSF, Peprotech) at 100 ng/mL. The protocol was approved by the Scientific Committee of Institut de Recerca de la Sida – IrsiCaixa and the Ethics Review Board of Hospital Germans Trias i Pujol. Buffy coats were purchased from the Catalan Banc de Sang i Teixits (http://www.bancsang.net/en/index.html). The buffy coats received were totally anonymous and untraceable and the only information given was whether or not they have been tested for disease. All donors provided informed consent at the time of blood extraction. The human cell lines HEK293-T, TZM-bl (AIDS Reagent Program, National Institutes of Health, Bethesda, MD) MDA-MB-468 (ATCC® HTB132™) and T47D (ATCC® HTB-133™) (American Type Culture Collection, ATCC) were cultured in Dulbecco’s modified Eagle’s medium (DMEM; Gibco, Madrid, Spain) supplemented with 10% heat inactivated FCS, 100 U/mL penicillin, and 100 ug/mL streptomycin. SAMHD1 knock-out TZM cells were generated by CRISPR/Cas9 technique as described elsewhere (22). 4.2. Drugs 33-Azido-3-deoxythymidine (zidovudine, AZT), nevirapine (NVP), 1-beta-D- Arabinofuranosylcytosine (AraC), 9-beta-D-Arabinofuranosyl-2-fluoroadenine (fludarabine), 2-Amino- 9-beta-D-arabinofuranosyl-6-methoxy-9H-purine (nelarabine), 2-Chloro-20 -deoxyadenosine (cladribine), 2-chloro-9-(2-Deoxy-2-fluoro-beta-D-arabinofuranosyl)adenine (clofarabine), 20 ,20 - Difluorodeoxycytidine (gemcitabine), 5-Fluoro-2-desoxyuridine (floxuridine), 5-Fluoropyrimidine- 2,4-dione (fluorouracil), pemetrexed and 40 -N-benzoylstaurosporine (midostaurin) were purchased from Sigma-Aldrich (Madrid, Spain). Palbociclib, ribociclib and abemaciclib were purchased from Selleckchem (Munich, Germany). 4-amino-10-methylfolic acid (methotrexate) was purchased from Eurodiagnosticos SL (Madrid, Spain). 4.3. Virus Envelope-deficient HIV-1 NL4-3 clone encoding IRES-GFP (NL4-3-GFP) was pseudotyped with VSV-G by cotransfection of HEK293T cells using polyethylenimine (Polysciences, Warrington, PA, USA) as previously described [44]. For the production of viral-like particles carrying Vpx (VLP-Vpx), HEK293T cells were cotransfected with pSIV3+ and a VSV-G expressing plasmid. Three days after transfection, supernatants were harvested, filtered and stored at −80 ◦ C. Viral stocks were concentrated using Lenti-X concentrator (Clontech, Mountain View, CA, USA). Viruses were titrated by infection of TZM cells followed by GFP quantification by flow cytometry. 4.4. Virus Infections MDMs were pretreated with VLP-Vpx for 4h before infection or left with fresh media as a control. Cells were then infected with VSV-pseudotyped NL4-3-GFP and drugs were added at the time of infection. Viral replication was measured two days later by flow cytometry, as well as measurement of the viability of cells by gating live vs. dead cells. The anti-HIV activity of the different compounds was determined by infection of cells in the presence of different concentrations of the drug and 50% effective concentrations (EC50 ) were calculated, as previously described [23]. Cancers 2020, 12, 713 15 of 19 4.5. Western Blot Cell extracts were prepared for Western blot protein analysis as described before [22]. Briefly, cells were rinsed in ice-cold phosphate-buffered saline (PBS) and extracts prepared in lysis buffer (50 mM Tris HCl pH 7.5, 1 mM EDTA, 1 mM EGTA, 1 mM Na3VO4, 10 mM Na β-glycerophosphate, 50 mM NaF, 5 mM Na Pyrophosphate, 270 mM sucrose and 1% Triton X-100) supplemented with protease inhibitor (Roche, Basel, Switzerland) and 1 mM phenylmethylsulfonyl fluoride. Lysates were subjected to SDS-PAGE and transferred to a PVDF membrane (ImmunolonP, Thermo, Waltham, MA, USA). The following antibodies were used for immunoblotting: anti-rabbit and anti-mouse horseradish peroxidase-conjugated secondary antibodies (1:5000; Pierce, Waltham, MA, USA; anti-human Hsp90 (BD Biosciences, Franklin Lakes, NJ, USA; 610418), anti-SAMHD1 (1:2500; ab67820, Abcam, Cambridge, UK) and anti-RRM2 (1:1000; ab115701, Abcam) and anti-phospho-CDK2 (Thr160; 2561), anti-CDK2 (2546), anti-CDK1 (77055) anti-Rb (9309) anti-phospho-Rb (Ser807/811, 9308) anti-E2F1 (3742), anti-phospho-SAMHD1 Thr592 (15038) (all 1:1000; Cell Signaling Technologies, Danvers, MA, USA). 4.6. Evaluation of Cytotoxicity Cytotoxicity measurements in cell lines were based on the viability of cells that had been treated or not with various concentrations of the different anticancer compounds. Cells were treated at the indicated doses of the test compounds for 3 days and the number of viable cells was quantified by a tetrazolium-based colorimetric method (MTT method) as described elsewhere [23]. The MTT assay measured the metabolic activity of cells, resulting in a very sensitive procedure to evaluate cell viability and cell proliferation, including the effect of cytostatic agents that slow or stop cell growth. 4.7. Evaluation of Drug Combination Drug combinations were evaluated using the combination index (CI)-isobologram equation, a method widely used in pharmacology to study drug interactions. Relative values of drug activity were used to calculate CI as implemented in the Compusyn software (Combosyn Inc., Paramus, NJ, USA) [45]. In brief, combination experiments were performed by using serial dilutions of each drug alone or a mixture of the two drugs evaluated, as recommended by the Chou–Talay method using a non-constant ratio combination [46]. CI was calculated for all combinations and those combinations, including concentrations of SAMHD1-activating drugs around calculated IC50 , were considered for quantification of drug combination effect. Drug combinations with CI < 1 were considered synergic [46]. 4.8. Immunohistochemistry Tissue sections from lung and pancreas tumor tissues (T2235188-1 and T2235152, respectively, Amsbio UK), were used to evaluate SAMHD1 expression in tumor tissues ex vivo. All immunohistochemical analyses were performed in the Histopatology core facility at Germans Trias i Pujol Research Institute. A polyclonal rabbit anti-SAMHD1 antibody (cat. no. 12586-1-AP, Proteintech, Rosemont, IL, USA) and an automated detection system were utilized. The specificity of the polyclonal antibody was previously tested by Western blot analysis in cell lines and by immunohistochemistry using paraffin-embedded normal tissue. Images were obtained in a Zeiss Axioskop 2 microscope using ZEN blue 2011 software. 4.9. Statistical Methods Data were analyzed with the PRISM statistical package. If not stated otherwise, all data were normally distributed and expressed as mean ± SD. p-values were calculated using an unpaired, two-tailed, t-student test. Cancers 2020, 12, 713 16 of 19 5. Conclusions Our results confirm and expand the number of drugs whose efficacy might be affected by SAMHD1 function, demonstrating the importance of SAMHD1 for targeted health therapies including nucleoside analogues such as treatments for cancer and viral infections. Moreover, we show that pharmacological modulation of SAMHD1 can significantly enhance current antimetabolite-based anticancer therapies. Nucleobase and nucleoside analogues are an important class of anti-cancer and antiviral drugs. Thus, understanding the clinical and molecular determinants of drug efficacy is paramount to improve the efficacy of anticancer treatment. Based on our findings, the development of robust SAMHD1 inhibitors and activators that can potentiate antimetabolite therapeutic regimens should become a priority. Supplementary Materials: The following are available online at http://www.mdpi.com/2072-6694/12/3/713/s1, Figure S1: Efficacy of the multi-kinase inhibitor midostaurin depends on SAMHD1 expression, Figure S2: Relative effect of the combination of palbociclib-fluorouracil and midostaurin-palbociclib, Figure S3: Expression of cell cycle proteins and SAMHD1 in the cell lines used, Table S1: Cytotoxic activity of evaluated drugs in the different cell lines. Author Contributions: Conceptualization, E.R.-M., M.M. and E.B.; Formal analysis, M.M. and E.B.; Investigation, M.C., E.F., I.J.E., R.B., E.G.-V., M.P. and L.G.-C.; Methodology, I.T. and A.M.-C.; Supervision, B.C.; Writing – original draft, E.B.; Writing – review & editing, E.R.-M. and E.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Instituto de Salud Carlos III, Fondo de Investigación Sanitaria (FIS) PI16/00103, PI17/00624 and CP14/00016 cofinanced by FEDER. EB is a research fellow from ISCIII-FIS (CP14/00016). EGV, MP, LG are research fellows from Generalitat de Catalunya AGAUR. RB is a research fellow from PERIS, Generalitat de Catalunya (PERIS SLT002/16/00059). IE is a research fellow from la Caixa Bank Foundation (LCF/BQ/IN18/11660017) cofunded by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 713673. Acknowledgments: We thank the National Institutes of Health (AIDS Research and Reference Reagent Program) for reagents. We thank the IGTP Histopathology Core Facility and staff for their contribution to this publication. Conflicts of Interest: The authors declare no conflict of interest. References 1. Goldstone, D.C.; Ennis-Adeniran, V.; Hedden, J.J.; Groom, H.C.; Rice, G.I.; Christodoulou, E.; Walker, P.A.; Kelly, G.; Haire, L.F.; Yap, M.W.; et al. HIV-1 restriction factor SAMHD1 is a deoxynucleoside triphosphate triphosphohydrolase. Nature 2011, 480, 379–382. 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