voruciclib (ME-522)

Voruciclib at doses up to 200 mg administered on 14 consecutive days on a 28-day cycle was well tolerated, with no DLTs observed, and no cardiovascular toxicities. There was no significant myelosuppression in pts with B-cell malignancies. As the safety profile suggested non-overlapping toxicities with venetoclax, the study is now evaluating voruciclib in combination with venetoclax in pts with R/R AML to exploit dual inhibition of Bcl-2 and Mcl-1.

To date, in Groups I and II combined, 1 AML pt achieved a morphologic leukemia-free state, 2 AML pts achieved stable disease (SD), and 3 pts with B-cell malignancies had a best response of SD, including 1 FL pt who achieved a 42% reduction in SPD, and 1 CLL pt with relapsed disease after chemoimmunotherapy, venetoclax, and ibrutinib who had stable blood counts. Enrollment at 200 mg will continue in an expansion cohort in AML. A forthcoming protocol amendment will evaluate VORU in combination with venetoclax in pts with relapsed AML to exploit the dual inhibition of Bcl-2 and Mcl-1.

AZD5991 is a highly selective BH3-mimetic that demonstrates high in vitro potency in MCL1-dependent cell lines with an IC50 <1 nM.7 We have demonstrated that direct MCL1 inhibition with AZD5991 disrupts survival of neoplastic B-cells in lymph-node mimicking conditions, induces mitochondrial dysfunction and cooperates with BCL2/X inhibitors in vitro and in vivo.8 A phase 1 clinical trial of AZD5991 alone or in combination with venetoclax in hematologic malignancies is ongoing (NCT03218683)...AMG-176 demonstrated synergy with venetoclax in AML models; however, it also caused cytopenias.9 AMG-176 was shown to induce apoptosis of CLL cells independent of prognostic markers and overcame the protective effect of stromal microenvironment.10 However, suppressive effects on hematopoiesis will likely become the dose- limiting factor in clinical trials of MCL1-targeting agents. The effect of MCL1 inhibition on cord blood-derived CD34+ cells was studied using a small-molecule inhibitor, S63845,11 resulting in almost complete depletion of human hematopoietic stem and progenitor cells, while mature blood cells survived normally...We have shown that SYK inhibition with entospletinib leads to downmodulation of MCL1 protein in CLL both in vitro and in the clinic.12,13 Treatment of CLL cells with entospletinib, but not other BCR-signaling inhibitors, led to a disruption of BAFF-BCR cross-talk and downmodulation of MCL1 mRNA and protein, thus implicating SYK in transduction of multiple pro-survival signals emanating from the tumor microenvironment.13 Entospletinib has shown promising clinical activity in CLL.12,14 It is currently being developed in myeloid malignancies.15 A novel dual SYK/BTK inhibitor luxeptinib also was shown to downregulate MCL1 and other pro-survival BCL2 proteins in CLL.16 Transcriptional cyclin-dependent kinases (CDK7/9) regulate activity of RNA polymerase, thereby controlling production of mRNA. Downmodulation of MCL1 has been considered a key mechanistic event accounting for the pro-apoptotic activity of CDK inhibitors in neoplastic B-cells.17,18 Pan-CDK inhibitors (flavopiridol, dinaciclib) demonstrate clinical efficacy in lymphoid malignancies. We and others reported that preclinical efficacy of AZ5576, a selective inhibitor of CDK9, and its clinical congener AZD4573 in lymphoid tumors depended on rapid downmodulation of MCL1, BFL1, and MYC.19–21 CDK9 inhibition can increase efficacy of BH3-mimetics, but safety of this combination will need to be explored.22,23 CDK9 inhibitors in development, in addition to AZD4573, include VIP152, CYC065, and voruciclib...Navitoclax is a small-molecule inhibitor of BCL2, BCLX, and BCL2L2; however, its use in CLL has been plagued by thrombocytopenia.24 AZD4320 is an alternative agent that co-targets BCL2/X...ABBV-155, an antibody-drug conjugate targeting BCLX, where BCLX is being used as a payload to avoid “off-tumor” effects, demonstrated promise in solid tumor models.26 Finally, as many microenvironment-driven pro-survival pathways converge on NFB, leading to induction of chemokines, cell cycle regulators, and BCL2 family proteins themselves, this transcription factor continues to be an attractive target in CLL...When combined with ibrutinib in murine CLL models, VAY-736 produced prolonged survival compared with either therapy alone.28 Alternative targets for therapeutic antibodies include ROR1 (cirmtuzumab), CD19 (tafasitamab), and others.29 As resistance to novel agents is becoming an unmet need in CLL, exploration of novel targets is of paramount importance...Multiple CDK inhibitors are currently being explored and have anti-tumor effects not restricted to MCL1 inhibition. CAR T cells and bi-specific antibodies have exceptional efficacy in lymphoid malignancies and thus are of high relevance in CLL.

However, 10–50% of newly diagnosed patients with AML may not respond to venetoclax and HMA or LDAC, and 3–15% patients may not respond to venetoclax with intensive or non-intensive chemotherapy.1–6 In addition, up to 40% of responding patients may relapse with low rates of response of 20% to salvage therapy and poor overall survival of 2 months after relapse.7 Clinical and biological factors associated with primary and acquired resistance to venetoclax include secondary AML, monocytic differentiation, complex cytogenetics, mutations in TP53, BAX, dependence on other anti- apoptotic proteins, altered metabolism of nicotinamide, fatty acids, and oxidative phosphortylation.3,8–14 Several novel inhibitors of BCL-2 are currently being tested in clinic, including BGB 11417, APG-2575, LP-108 and others...There is strong pre-clinical rationale for targeting MCL-1 alone as well as in conjunction with BCL-2 inhibition in AML.15 Recently several selective and highly potent MCL-1 inhibitors have entered pre-clinical and clinical development including S63845, AZD5991, AMG397, and others. Questions remain regarding the therapeutic window of these inhibitors given the important physiologic role of MCL-1 in vital organs and early reports of cardiac adverse events from the AMG176 phase 1 trial.15,16 Multiple pre-clinical studies have expectedly shown synergism between BCL- 2 and MCL-1 inhibition making it a promising path for clinical development of these agents.17,18 Multifactorial challenges in design of specific MCL-1 inhibitors also led to interest in compounds which downregulate MCL-1 expression. Cyclin dependent kinase (CDK) inhibitors including alvocidib, dinaciclib, voruciclib are in various stages of evaluation. Although addition of alvocidib to intensive chemotherapy improved response rates but failed to improve event-free or overall survival.19 Novel CDK inhibitors are currently in early phase trials including AZD4573, CYC065, TG02-101, and others. Inhibition of Nedd8 activating enzyme has complex repercussions for the intrinsic apoptotic pathway with eventual increase in Noxa leading to MCL-1 neutralization.20 Pevonedistat has shown promising early results in AML and myelodysplastic syndrome and is being investigated multiple clinical trials for solid tumors as well. BCL-xL Inhibition Another anti-apoptotic protein BCL-xL had been long recognized as a potential therapeutic target in AML, in particular AML from preceding MPN and AML recurrent post venetoclax failure, but toxicity of earlier inhibitors precluded clinical development.21–23 Recently, AZD0466, a dual BCL-2/xL inhibitor with a favorable therapeutic index and robust activity has been developed and is undergoing pre-clinical development and planned for phase iin hematological malignancies.24 Targeting the Extrinsic Apoptosis Pathway Inhibitor of apoptosis protein (IAP) inhibition: X-linked IAP (XIAP), cellular IAP (cIAP) and survivin have been of long- standing interest in AML. Prior clinical trials with XIAP inhibitor AEG35156, cIAP targeting agent birinapant, and survivin targeting agent LY2181308 have not succeeded in clinc.16,25 ASTX660 is a dual antagonist of XIAP and cIAP which is currently being investigated in phase 1/2 trials in solid tumors and in combination with HMA in relapsed or refractory AML.26,27 TRAIL Agonism Agonists of the TNF-related apoptosis-inducing ligand (TRAIL) receptors have been tested in AML with low response rates.28,29 Previous agents have had limited success in part due to suboptimal clustering of TRAIL receptors.30 Novel antibodies against TRAILR1 and TRAILR2 including an IgM molecule IGM-8444, a tetravalent compound INBRX-109, and HLX56 are currently in phase 1 trials and preclinical data suggests potential synergy with venetoclax.31 FLIP Inhibition FLICE-like inhibitor protein (FLIP or CFLAR) is a key regulator of the death-inducing signaling complex (DISC) involved in the extrinsic apoptotic pathway...This can be augmented by inhibiting p53 degradation via MDM2, which is often upregulated in AML.34 Idasanutlin in combination with venetoclax showed anti- Figure 1 leukemic activity in the dose finding stage in R/R AML.35 Several other inhibitors of MDM2 and dual MDM2/X inhibitors are currently in various stages of pre-clinical and clinical development including HDM-201, KRT-232, BI-9078282, and others.34 Conclusions Opportunities to target the apoptosis machinery in AML has considerably evolved in the last decade. While venetoclax heralded a paradigm shift for patients, we are now faced with challenges in patients who relapse or remain refractory. We have novel clinical stage compounds to methodologically target different facets of the apoptotic pathway and optimize novel combinations with the goal to improve the cure rates in AML patients.

Collectively, these data demonstrate that voruciclib inhibition of CDK9 leads to reduced phosphorylation of MYC on Ser 62 followed by a decrease in total MYC protein in MIA PACA-2 cells and inhibition of growth in multiple KRAS mutant cancer cell lines in vivo and in vitro. This suggests that voruciclib could be an attractive therapeutic option for cancers driven by KRAS-MYC.

The search also included clinical trials registered in clinicaltrials.gov.Expert opinion: The use of BTK and PI3Kδ inhibitors (ibrutinib, acalabrutinib, idelalisib and duvelisib) and BCL-2 antagonist venetoclax have changed the treatment strategy of CLL. Several clinical trials with novel, unapproved agents are currently ongoing. Their findings should define the role of these novel drugs in the treatment of patients with previously untreated and relapsed or refractory CLL.

Accordingly, an every other day schedule of the CDK9 inhibitor is effective in vitro and in vivo in enhancing the efficacy of venetoclax. Our preclinical data provide a rationale for an intermittent drug administration schedule for the clinical evaluation of the combination treatment for AML.

Accordingly, an every other day schedule of the CDK9 inhibitor is effective in vitro and in vivo in enhancing the efficacy of venetoclax. Our preclinical data provide a rationale for an intermittent drug administration schedule for the clinical evaluation of the combination treatment for AML.