BMS-265246 | Gsk J4

Cyclin-dependent Kinases as Emerging Targets for Developing Novel Antiviral Therapeutics

Lucia Gutierrez-Chamorro, Eudald Felip, Ifeanyi Jude Ezeonwumelu, Mireia Margelí, and Ester Ballana
1AIDS Research Institute-IrsiCaixa and Health Research Institute Germans Trias i Pujol (IGTP), Hospital Germans Trias i Pujol, Universitat Autònoma de Barcelona, 08916, Badalona, Spain
2B-ARGO (Badalona Applied Research Group in Oncology) and Health Research Institute Germans Trias i Pujol (IGTP), ICO- Badalona, Hospital Germans Trias i Pujol, Universitat Autònoma de Barcelona, 08916, Badalona, Spain

Besides its prominent role in cell proliferation, cyclin-dependent kinases (CDKs) are key players in viral infections as both DNA and RNA viruses modify CDK func- tion to favor viral replication. Recently, a number of speciﬁc pharmacological CDK inhibitors have been developed and approved for cancer treatment. The repurposing of these speciﬁc CDK inhibitors for the treatment of viral infectionsmay represent a novel effective therapeutic strategy to combat old and emergentviruses. In this review, we describe the role, mechanisms of action, and potential of CDKs as antiviral drug targets. We also discuss the current clinical state of novel speciﬁc CDK inhibitors, focusing on their putative use as antivirals, espe- cially against new emerging viruses.

Introduction
Despite the recent advances in controlling viral pathogens, most viral infections still lack speciﬁc treatment. In fact, the approval of new direct-acting antivirals (DAAs) is not paralleled with the pressing need of effective therapeutic strategies to combat ‘old’, emergent, and re-emergent viruses. Drug repurposing is a particularly effective strategy that has been gaining a foothold in the research scape to hasten the development of new drugs to ﬁght viral infections by identifying new uses for already-in-use drugs with pre-existing clinical data [1,2]. However, drug repurposingis still a potential approach and more research is needed for its implementation in clinics.
The development of effective therapeutics is subordinated to the understanding of molecular mechanisms underlying virus replication and pathogenesis and virus–host interactions, which re- main puzzling for several known viral pathogens and also represent the main challenge for com- batting emerging viruses. In this context, evaluating the repurposing potential of existing drugscan represent an effective strategy for the development of ready-to-use antiviral agents as well as for the identiﬁcation of new pathways and targets for intervention.
CDKs are protein kinases that play important roles in the control of cell division and modulation of transcription in response to several extra- and intracellular signals. Several instances of dysregu- lation in the function of CDKs and their corresponding cyclin partners have been described in can- cer as it could lead to uncontrolled cell division and proliferation [3]. On the other hand, CDKs are also key players in several viral infections as both DNA and RNA viruses are able to modify cellular CDKs to generate a favorable environment for viral replication, either by dysregulating cell-cycle progression or by overcoming cellular restriction factors [4].
Over the last few decades, CDKs have been considered attractive targets for drug development. Despite the important limitations and clinical failures of the ﬁrst and second generations ofpharmacological CDK inhibitors (PCDKis), speciﬁc PCDKis targeting CDK4 and CDK6 (CDK4/6) have shown signiﬁcant clinical activity in HER-2-negative metastatic breast cancer with an ac- ceptable toxicity proﬁle, demonstrating the feasibility of targeting CDK function as a therapeutic strategy [5,6]. Indeed, treatment with pharmacological CDK4/6 inhibitors can cause cell deathand tumor size regression, suggesting additional mechanisms beyond simple cell-cycle arrest and pointing towards an improved immune response as the main mechanism responsible for in- creasing the clinical beneﬁt resulting from the inhibition of CDK4/6, a process also highly relevantfor ﬁghting viral infections [1].
Here, after a brief overview of CDK function, we highlight the role of CDKs in viral infections. We also discuss the current clinical state of novel speciﬁc PCDKis, focusing on their role in ﬁghting viral infections, speciﬁcally those caused by newly emerging viruses, such as the SARS-CoV-2 virus responsible for the recent coronavirus outbreak.

An Overview of the Role of CDKs
In human cells, more than 20 CDKs have been described: CDK1 to CDK6 drive cell-cycle pro- gression, whereas CDK7 to CDK12 are mainly involved in the regulation of gene transcription. CDK activity depends on the binding of a regulatory subunit – a cyclin. Although cyclins do not have enzymatic activity on their own they are among the most important core cell-cycle regula- tors. By partnering with a speciﬁc CDK, the cyclin activates the former and subsequently drives the events of the cell cycle (Figure 1).
The principal regulatory mechanisms of the cell cycle by CDKs are very well established (Box 1). Apart from cell-cycle regulation, CDKs can inﬂuence different transcription processes, both indi- rectly through their inﬂuence on transcription factors, such as E2F and Forkhead Box M1 (FoxM1), or more directly through regulation of RNA polymerase II (RNA Pol-II)-dependent transcription. Among them, CDK7 (which, together with cyclin H, forms the human transcription factor II or TFIIH), CDK8 (which is a component of the Mediator complex, a multiprotein complex that functions as a transcriptional coactivator), and CDK9 (which is a part of the positive transcrip-tion elongation factor or pTEFb) are critical transcription factors [7–9]. In addition to cell-cycle reg- ulation and transcription, CDKs have been implicated in a series of other processes that do not depend exclusively on the kinase’s activity. These noncanonical functions have been discussed and reviewed elsewhere [10,11], and those impacting virus replication will be further discussedin subsequent sections.
CDK activities are not only controlled by binding to cyclins but also by other mechanisms such as phosphorylation or binding to CDKis. Two CDKi gene families have been described in mammals:the INK4 family and the Cip/Kip family. Members of the INK4 family, p16INK4a, p15INK4b, p18INK4c, and p19INK4g, speciﬁcally inhibit the activity of CDK4/6; while Cip/Kip family members, p21Cip1, p27Kip1, and p57Kip2 interact with a broader range of both cyclins and CDKs to modulate their functions [12,13] (Figure 1).

Viral Manipulation of the Cell Cycle by CDKs
Viruses rely on the host cell for resources to create a favorable environment for their own replica- tion. Irrespective of the strategy that a virus uses for its propagation, all viruses have evolved ap- propriate and complementary mechanisms for the manipulation and subversion of the many defensive mechanisms of the host’s innate and acquired immune system [17]. Viruses use a va- riety of strategies to usurp and control cellular activities, with the manipulation of the host cell cyclebeing one of the most frequent. The complex and interactive nature of intracellular signaling path- ways controlling cell division affords many opportunities for viral manipulation strategies that

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Figure 1. Schematic Representation of Regulation of Cyclin-Dependent Kinase (CDK) Activity during the Cell Cycle. Progression through the distinct cell cycle phases involves the successive formation, activation, and subsequent inactivation of CDKs, accomplished by the sequential binding and phosphorylation of each CDK. Entry into the cell cycle is controlled by CDK4 and CDK6 containing D-type cyclins that phosphorylate the Retinoblastoma proteins (pRb), releasing E2F transcription factors, which subsequently control the transcription of genes needed for entry into the S phase. Progression through the S phase is controlled by CDK2-cyclinE/cyclinA complexes. Binding of cyclin-dependent inhibitors(CDKis) to CDKs speciﬁcally inactivates either CDK4/6 or CDK2 (INK4 family and Cip/Kip family respectively). Finally, theG2-to-M transition is carried out by cyclin A and cyclin B coupled to CDK1, a step that is inhibited by the Wee1 and checkpoint1 (CHK1) kinases depend on the speciﬁc characteristics of the different virus families, although several fundamental similarities exist between RNA viruses, and some parallels can be made with DNA viruses [18] (Table 1). However, as DNA viruses generally replicate in the nucleus they have been more exten- sively characterized in regard to their effects on the cell cycle.
Here, we focus on the modiﬁcation of cellular CDKs, and related host factors, by viruses that aim to dysregulate cell-cycle progression or cell-cycle-controlled cellular restriction factors to overcome host defenses. We provide detailed examples of some of the most representative mechanisms. A detailed description of the relationship between other cellular proteins involved in cell-cycle regulation and viral replication can be found elsewhere [4,18].

Small DNA Viruses
Despite the wide variety of small DNA viruses, each of them with their unique characteristics, these viruses share common features with respect to their replication cycle. As they code for few or no DNA-replication proteins, they depend strictly on active cellular proteins for their own replication. Therefore, small DNA viruses replicate only when the infected cell progresses into the S phase. Some of them, such as parvoviruses, depend on spontaneous progression to the S phase by infected cells. In contrast, papillomaviruses promote progression of the cell cycle into an S-phase-like state, necessary for productive viral replication and characterized by high CDK activity [4,17]. In order to achieve this S-phase-like state, most of these viruses encode different proteins that modify the activities of cellular cell-cycle regulators, such as the transcrip- tional transactivator X protein (HBx), the oncoproteins E6 and E7, or the large and small tumor(T) antigens, encoded by the hepatitis B virus (HBV), the human papilloma virus (HPV), and simian virus 40 (SV40), respectively [19,20]. Speciﬁcally, all of these viral proteins normally bind and inacti- vate ppRb and CDKi, such as p27Kip1, while they also disrupt cyclin D1/CDK4 complexes and induce expression of the CDKi p16INK4A [21,22]. In addition, these proteins repress p53 function,induce the expression of cyclin A/E-CDK2 complexes, and most recently it has been demonstrated that they positively regulate CDK9 [23–25]. In turn, all of these actions inhibit G1-speciﬁc CDK activities but promote progression into an S-phase-like state.

Large DNA Viruses
In contrast to small DNA viruses, larger viruses encode many of the proteins required for their replication and can elicit cell-cycle arrest so that competition for cellular DNA replication resources is limited. A representative example is the family Herpesviridae. Despite cell-cycle arrest and cellular DNA replication, almost entirely blocked during their lytic infection phase, herpesviruses sustain an active cellular metabolic state and modulate the host’s cell cycle to achieve a cellular state with highly active CDKs, called the S-phase-like state [26]. Although each of the three herpesvirus subfamilies (Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae) has their own mechanisms to achieve this optimal cellular state for viral replication, there are some common patterns, such as the degradation of p53 and other CDKis [27]. The overall elevation of CDK activities consecutively activates the transcription of many proteins involved in cellular DNA synthesis and cell-cycle progression. In addition, while the majority of viral families manipu- late the activity of cellular kinases, the Herpesviridae family encodes genes that are functional orthologs of cellular CDKs, the so-called CHPKs (conserved herpesvirus-encoded protein kinases) that contribute to the creation of the pseudo-S-phase environment. Apart from CHPKs, each subfamily of herpesviruses encodes other proteins that contribute towards themaintenance of the S-phase-like state. Examples of these proteins include BGLF4 of Epstein– Barr virus (EBV, subfamily Gammaherpesvirinae), K-cyclin of Kaposi’s sarcoma-associatedherpesvirus (KSHV, subfamily Gammaherpesvirinae), or UL97 of human cytomegalovirus (HCMV, subfamily Betaherpesvirinae), all of which activate CDKs [17].
Moreover, some of the small and large DNA viruses mentioned in the previous text are known human oncogenic viruses. These include EBV, HBV, human T-lymphotropic virus 1 (HTLV-1), human papillomaviruses, hepatitis C virus (HCV), KSHV, and Merkel cell polyomavirus. Despite the fact that they are very diverse, they promote tumorigenesis in humans by encoding a seriesof oncoproteins that dysregulate cell-cycle proteins, mainly those driven by the p53 and pRb pathways. This dysregulation, in turn, results in progression of the cell cycle into an S-phase- like state (reviewed in [28]). The association between CDKs/CDKi with viral oncoprotiens has been extensively characterized [27]. There is mounting evidence that shows parallels between virus-induced cancer cells and nonviral cancers, in that functional alteration of CDKs and CDKis provides the optimal conditions for malignant cells, allowing them to proliferate vigorously. The in-depth description of these functions highlights the key role of CDKs in viral replication but also their putative application as novel therapeutic approaches against viral infections.

Single-Stranded RNA Viruses
Single-stranded RNA viruses that replicate mainly in the cytoplasm affect the host’s cell cycle by inducing cell-cycle arrest, either at G2/M phase or at G0/G1 phase. The family Coronaviridae is a very good example of this category as several members of this family use both of these strategies.
However, knowledge of the speciﬁc mechanisms underlying this cell-cycle regulation is limited, especially for human coronaviruses, as most studies were done with prototypic coronavirusessuch as the infectious bronchitis virus (IBV), an avian coronavirus, and with the mouse hepatitis virus (MHV). The IBV-induced cell-cycle arrest at the G2/M phase is driven by the modiﬁcation of several cell-cycle-regulatory proteins that – coupled with the downregulation of cyclins D1 and D2 (G1-regulatory cyclins) – induced aberrant cytokinesis in which the cells underwent nuclear, but not cytoplasmic, division. By comparison, MHV-induced cell-cycle arrest occurs at the G0/G1 phase, through the downregulation of CDK4/6 and their corresponding cyclins [29,30].
Due to the recent SARS-CoV-2 outbreak, large amounts of data are being published, shedding some light on cell-cycle modiﬁcations by these viruses [31]. In order to ensure advantageous con- ditions for viral replication, SARS-CoV-2 infection induces a strong regulation of CDK signaling pathways, especially by reducing CDK1/2 activity, leading to an S/G2-like phase arrest, similar to that produced by other RNA viruses [32]. Although the mechanism is poorly understood, pre- vious studies with other coronaviruses have suggested that the nucleocapsid protein (N protein),which is highly conserved among these viruses, has a key role in this process [33]. In the case of SARS-CoV-1, the N protein inhibits the activity of the cyclin D-CDK4 and cyclin A/E-CDK2 complexes. The cyclin D-CDK4 complex is inhibited by direct binding of the N protein to cyclinD. The activity of the cyclin A/E-CDK2 complexes is inhibited both by indirect downregulation of the levels of CDK2, cyclin E, and cyclin A or by direct binding to the cyclin A-CDK2 complex [33]. Recent evidence has also shown an increase in the phosphorylation of CDK2 at positions T14 and Y15 by SARS-CoV-2, preventing premature entry into mitosis [32].
Likewise, other single-stranded RNA viruses (that are also important human pathogens), such as inﬂuenza A viruses, also arrest the cell cycle in a similar way by inducing cell-cycle arrest at G0/G1, through physical interaction between the viral matrix protein, M2, and cyclin D3. This is subse- quently followed by cyclin D3 relocalization and degradation [34,35].

Retroviruses
The relationship between retroviruses and the cell cycle varies among the different subfamilies; however, in most of them, modiﬁcations of cellular CDK activity may dysregulate cell-cycle pro- gression in order to optimize the cellular requirements for viral replication. In human immunodeﬁ- ciency virus (HIV) infection, the accesory viral protein R (Vpr) induces potent G2/M arrest in cyclingcells, although the underlying molecular mechanism still remains poorly understood [36]. Nevertheless, seven cellular CDKs have been described that mainly affect viral transcription either directly or indirectly through the viral restriction factor SAMHD1 (Figure 2, Box 2). The role of

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Figure 2. The Role of Cyclin-Dependent Kinases (CDKs) in the HIV Replication Cycle. This schematic diagram sumarizes the HIV replication cycle and the different steps where CDKs play a role. After entry into the host cell, the viral RNA is reverse transcribed into DNA. At this point, the antiviral activity of sterile alpha motif and histidine-aspartic acid domain-containing protein 1 (SAMHD1) is inactivated by its phosphorylation by several CDKs, such as CDK1, CDK2, and CDK6. The viraDNA enters the nucleus and is transcribed by the host’s normal transcription machinery into multiple copies of new HIV RNA. During this process, CDK7 and CDK9 play an important role as part of the RNA-pol II initiation and elongation complexes: CDK7 is part of TFIIH (human transcription factor II), and CDK9 and its cyclinpartner, T1, constitute the RNA-pol II elongation factor or pTEFb. CDK8, as part of the Mediator complex, also contributes to the transcription of the viral genome. Next, CDK11 and CDK13 play major roles in processing the viral transcript. CDK11, by its association with the multiprotein TREX/THOC, facilitates 3′-end processing of the viral transcript, while CDK13 is normally involved in RNA splicing. A few copies of this RNA are incorporated into new virus particles, whereas others are used as messenger RNA for the production of new viral proteins. Viral proteins assemble together with the genomic viral RNA to form a virus particle that is released from the cell.
CDKs in HIV-1 transcription is mainly driven by their function as part of the RNA-pol II initiation and elongation complexes, such as CDK7, which is part of TFIIH, or CDK9 and its cyclin partner, cyclin T1, that constitute the RNA-pol II elongation factor or pTEFb [37,38]. CDK2 phosphorylates, and thereby regulates, the activity of several proteins involved in HIV-1 replication, including CDK7,
CDK9, and the viral Tat protein [39,40]. CDK11 facilitates 3′-end processing, increasing cleavage and polyadenylation of the viral transcript by its association with the multiprotein TREX/THOC com-
plex (transcription and export complex/THO complex) that prevents DNA damage and regulates the cell cycle by ensuring optimal gene expression [41]. Finally, CDK13, which is normally involved in RNA splicing, and CDK8, which is part of the Mediator complex, have also been suggested to contribute to RNA-pol II-driven transcription of the viral genome [42–44].

Pharmacological CDK Inhibitors
Discovery and development of PCDKis has long been an interest in academic research and the pharmaceutical industry. Since the late 1980s, several PCDKis have been developed as potential cancer therapeutics and tested in numerous clinical trials [5,6].
Most PCDKis are ATP-competitive, interacting with CDKs within their ATP site. However, innova- tive approaches towards the development of novel, non-ATP-competitive PCDKis that act through other mechanisms are currently under research.
The ﬁrst PCDKis that were clinically developed, referred to as pan-PCDKis, were relatively non- speciﬁc multi-CDKis that target multiple CDKs as well as other kinases. Examples of these ﬁrst PCDKis are ﬂavopiridol and roscovitine [5]. Flavopiridol has been shown to inhibit CDK1, CDK2,CDK4, CDK6, CDK7, and CDK9, with substantial in vitro activity; while roscovitine inhibits CDK1, CDK2, and CDK5 but also RNA synthesis, possibly through the inhibition of CDK7 and CDK9 [52,54,55]. Currently, there are certain pan-PCDKis under investigation, such as roniciclib, a cyclin B-CDK1, cyclin E-CDK2, cyclin D1-CDK4, and cyclin T1-CDK9 inhibitor and JNJ-7706621, a dual-speciﬁc CDK1/2 and AURKA/B kinase inhibitor [56–58].
The important limitations, that is, lack of adequate balance between efﬁcacy and safety, together with clinical failures involved with the ﬁrst-generation PCDKis, prompted the development of more selective inhibitors with improved potency (milciclib, dinaciclib, CYC-065, and AT7519). This second generation of optimized pan-PCDKis have also demonstrated limited clinical beneﬁts as anticancer agents [59–61]. Of these second-generation PCDKis, dinaciclib, a highly potent inhibitor of CDK1, CDK2, CDK5, and CDK9, with less activity against CDK4, CDK6, CDK7,and CDK12, has been extensively studied in the clinic [62].
Nowadays, research is moving rapidly towards the development of more speciﬁc PCDKis. The inhibitors that have progressed the furthest, and have now entered clinical use, are those speciﬁcally targeting CDK4/6 [63]. These selective pharmacological CDK4/6 inhibitors not only demonstrate clinical beneﬁts but also fewer toxicities than previous PCDKis. To date, three pharmacological CDK4/6 inhibitors – palbociclib, ribociclib, and abemaciclib – have been approved by the UnitedStates Food and Drug Administration (FDA) and by the European Medicines Agency (EMA) to be used in combination with hormone therapy for metastatic breast cancer treatment. Other pharma- cological CDK4/6 inhibitors under development include trilaciclib (G1T28), lerociclib (G1T38), and tiviciclib (SHR-6390).
The molecular mechanism of action of palbociclib, ribociclib, and abemaciclib is relatively well described. As most PCDKis, they are ATP-competitive drugs that inhibit CDK4 and 6 by binding to the ATP clefts of these molecules. Their effect on inhibiting cell growth is achieved by blockingthe phosphorylation of pRb, leading to G1 cell-cycle arrest [63]. However, their effectiveness is also partially explained because of their modulation of the host’s immune system. This immuno- modulatory activity of PCDK4/6 inhibitors, that may be highly relevant in the ﬁght against viral infections, has been recently reported, showing that these inhibitors not only promote tumorcell-cycle arrest but also induce an immune response. Recently, different studies have shown that pharmacological CDK4/6 inhibitors enhance antigen presentation and increase levels of double-stranded RNA and type III interferon molecules [64]. Pharmacological CDK4/6 inhibitors can also enhance antitumor immune response by upregulating the activity of NFAT and the level of cytokines (IL-2) in the effector CD8+ T cells and promote expression of PD-L1, causing tumor evasion. Therefore, it has been hypothesized that anti-PD-1 immunotherapy could synergize with antitumor activities of pharmacological CDK4/6 inhibitors to further enhance immune activation [65–67], a process that might also be relevant to ﬁght viral infections.
Given the involvement of CDKs in multiple cellular processes, and the success of selective phar- macological CDK4/6 inhibitors, the development of selective inhibitors targeting other CDKs has been encouraged. Recently, intensive screening and drug design has led to the identiﬁcation of PCDKis that are not directly involved in cell-cycle regulation. It has been recently reported that limiting the function of some CDKs as transcriptional coactivators could impair the synthesis of tumor mRNAs without affecting the transcription of housekeeping genes. Here, CDK7, CDK8, CDK9, and CDK12 have been considered as viable and promising therapeutic targets [68–72]. In the coming years, different selective PCDKis, in combination therapy or monotherapy, maybe introduced into the clinics against a range of different diseases such as cancer, viral infections, or neurological disorders.

CDKs as Therapeutic Targets for Viral Infections
Although some of the ﬁrst-generation PCDKis showed modest antiviral activity, they were not considered feasible therapeutic options for viral infections. Nevertheless, roscovitine, one of the ﬁrst PCDKis developed, has the ability to inhibit replication of a broad range of viruses, even in nondividing cells, and continues to be considered as a potential clinical drug. HCMV, HSV-1/2,varicella zoster virus (VZV), EBV, human adenovirus, HIV, and HTLV are all susceptible in vitro to roscovitine [73] (Table 2). In addition, alsterpaullone, a CDK1, CDK2, and CDK5 inhibitor, is a potent inhibitor of HIV-1 transcription [74]. The advent of a newer generation of more speciﬁc PCDKis is prompting their application as novel therapeutic options for viral infections [1,17]. In the last 10 years, various speciﬁc PCDKis with variable selectivity have demonstrated antiviral activity either in wild-type or multidrug-resistant viral strains. As some of them have been demon- strated to be dependent on SAMHD1, this protein may represent a potentially new therapeutic target for PCDKis (Box 2). Furthermore, other speciﬁc PCDKis with proven in vitro antiviral efﬁcacyinclude dinaciclib, PHA-690509, and FIT-039; they inhibit multiple viruses, including inﬂuenza Avirus (IAV), Zika virus, and HBV, respectively [53,75,76] (Table 2).
In reference to coronaviruses, the huge impact of the current COVID-19 pandemic, which is caused by the novel SARS-CoV-2, is encouraging the design of drug-repositioning studies, in- cluding the repurposing of PCDKis for the treatment of SARS-CoV-2 infections. A study of global changes in kinase activity during SARS-CoV-2 infection noted a severe downregulation of CDK signaling pathways, indicating the potential for pharmacological inhibition of CDKs in COVID-19 therapy [32]. In order to accelerate treatment options, several FDA-approved drugs have been screened to identify candidates not only as antiviral drugs but also with the potential to reverse pulmonary insufﬁciency elicited by SARS-CoV-2, which can be life-threatening [77]. Amongover 3000 drugs screened, the pharmacological CDK4/6 inhibitor, abemaciclib, was listed ashaving potential antiviral activity against SARS-CoV-2 [78]. Furthermore, its inhibitory activity against spike-mediated entry in the earlier SARS and MERS viruses could contribute to the devel- opment of effective drug combinations for SARS-CoV-2 infection, considering the similarity of the spike protein (S) between SARS-CoV, MERS-CoV, and SARS-CoV-2. However, the associated toxicity of abemaciclib at its effective concentrations against these coronaviruses may limit its usein clinics [79]. Using a computation-based workﬂow, a PCDKi, palbociclib, has been described among a subset of FDA-approved drugs to potentially have the capacity to bind to the principal
SARS-CoV-2 protease MPRO. As this is only a theoretical model, additional experiments should be performed in order to conﬁrm these interesting yet predictive data [80]. Dinaciclib has also exhibited potent anti-SARS-CoV-2 activity. However, its hematological toxicities may also compromise its potential use [81]. Furthermore, some PCDKis have anti-inﬂammatory and cytokine-inhibitory properties, which may help to reduce pulmonary insufﬁciency caused by SARS-CoV-2 infection. This is the case for ﬂavopiridol, another PCDKi that has also been considered as a candidate for the treatment of COVID-19 [82]. Despite the promising nature of these results, there is still limited evidence for the possible beneﬁcial effect of PCDKis against SARS-CoV-2.

Concluding Remarks
At present, an important medical challenge is the lack of speciﬁc treatments for many viral infec- tions. The pressing need for effective and affordable therapeutic strategies to combat old but also emergent viruses increases the interest in drug repurposing as a feasible alternative for the iden- tiﬁcation of novel antivirals. In this context, a series of speciﬁc PCDKis that have been developedand approved as anticancer drugs in the last 5 years represent a promising antiviral strategy against a wide range of viruses that require CDKs for their replication. In this review we have pro- vided an overview of the function of CDKs, highlighting their interaction with distinct viruses and their prominent role in the viral life cycle, in addition to their canonical role in cell proliferation.
We have also discussed the current clinical state of novel speciﬁc PCDKis, focusing on their putative use as antivirals, either to ﬁght well characterized viruses or newly emerging viruses for which no effective treatments exist. Collectively, there is no doubt about the potential feasibilityof PCDKis as antiviral therapeutics, although their clinical value has still to be proven in trials (see Outstanding Questions). Future research is warranted to evaluate the potential of PCDKis as antivirals in relevant clinical settings and to unravel the mechanisms underlying speciﬁc CDK–virus interactions.

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