Identification and Characterization of ACP-5862, the Major Circulating Active Metabolite of Acalabrutinib: Both Are Potent and Selective Covalent Bruton Tyrosine Kinase Inhibitors

Acalabrutinib is a covalent Bruton tyrosine kinase (BTK) inhibitor approved for relapsed/refractory mantle cell lymphoma and chronic lymphocytic leukemia/small lymphocytic lymphoma. A major metabolite of acalabrutinib (M27, ACP-5862) was observed in human plasma circulation. Subsequently, the metabolite was purified from an in vitro biosynthetic reaction and shown by nuclear magnetic resonance spectroscopy to be a pyrrolidine ring-opened ketone/amide. Synthesis confirmed its structure, and covalent inhibition of wild-type BTK was observed in a biochemical kinase assay. A twofold lower potency than acalabrutinib was observed but with similar high kinase selectivity. Like acalabrutinib, ACP-5862 was the most selective toward BTK relative to ibrutinib and zanubrutinib. Because of the potency, ACP-5862 covalent binding properties, and potential contribution to clinical efficacy of acalabrutinib, factors influencing acalabrutinib clearance and ACP-5862 formation and clearance were assessed. rCYP (recombinant cytochrome P450) reaction phenotyping indicated that CYP3A4 was responsible for ACP-5862 formation and metabolism. ACP-5862 formation K_m (Michaelis constant) and V_max were 2.78 μM and 4.13 pmol/pmol CYP3A/min, respectively. ACP-5862 intrinsic clearance was 23.6 μL/min per mg. Acalabrutinib weakly inhibited CYP2C8, CYP2C9, and CYP3A4, and ACP-5862 weakly inhibited CYP2C9 and CYP2C19; other cytochrome P450s, UGTs (uridine 5'-diphospho-glucuronosyltransferases), and aldehyde oxidase were not inhibited. Neither parent nor ACP-5862 strongly induced CYP1A2, CYP2B6, or CYP3A4 mRNA. Acalabrutinib and ACP-5862 were substrates of multidrug resistance protein 1 and breast cancer resistance protein but not OATP1B1 or OATP1B3. Our work indicates that ACP-5862 may contribute to clinical efficacy in acalabrutinib-treated patients and illustrates how proactive metabolite characterization allows timely assessment of drug-drug interactions and potential contributions of metabolites to pharmacological activity. SIGNIFICANCE STATEMENT: This work characterized the major metabolite of acalabrutinib, ACP-5862. Its contribution to the pharmacological activity of acalabrutinib was assessed based on covalent Bruton tyrosine kinase binding kinetics, kinase selectivity, and potency in cellular assays. The metabolic clearance and in vitro drug-drug interaction potential were also evaluated for both acalabrutinib and ACP-5862. The current data suggest that ACP-5862 may contribute to the clinical efficacy observed in acalabrutinib-treated patients and demonstrates the value of proactive metabolite identification and pharmacological characterization.

Abstract 13: Structure elucidation, metabolism, and drug interaction potential of ACP-5862, an active, major, circulating metabolite of acalabrutinib

Acalabrutinib (Calquence®) is a potent, selective, orally administered, covalent inhibitor of Bruton tyrosine kinase (BTK) that received accelerated approval for relapsed/refractory mantle cell lymphoma from the US FDA in October 2017. Profiling of acalabrutinib metabolites in human plasma revealed a late-eluting, +16 Da metabolite circulating at concentrations higher than parent drug. Metabolite regiochemistry could not be determined by mass spectrometry. In vitro metabolism and preparative HPLC was used to generate a pure sample of the metabolite for structural characterization by NMR. Confirmatory chemical synthesis revealed a pyrrolidine ring-opened ketone. The structure of the metabolite, designated ACP-5862, and a smaller -2 Da peak, identified as dehydropyrrolidine, M25, inferred a common carbinolamide intermediate in their genesis. Both metabolites retained the butynamide electrophile responsible for the inactivation of BTK. In vitro studies on the inhibition of BTK and related Tec and Src kinases revealed that ACP-5862 was active against BTK with similar selectivity and potency to acalabrutinib (Kaptein et al, 2019) This work then investigated the in vitro metabolism and drug transport features of acalabrutinib, and the metabolite ACP-5862, to establish the potential for clinical drug-drug interactions (DDI) via CYPs, UGTs and drug transporters. CYP reaction phenotyping indicated CYP3A4 was responsible for both the formation and further metabolism of ACP-5862. Km and Vmax values for the formation of ACP-5862 using rCYP3A4 were 2.78 μM and 4.13 pmol/pmol CYP/min, respectively. The in vitro intrinsic clearance of ACP-5862 was 23.6 μL/min/mg. Acalabrutinib weakly inhibited CYP2C8, CYP2C9 and CYP3A4 in vitro, and ACP-5862 weakly inhibited CYP2C9 and CYP2C19, with no inhibition of CYP1A2, CYP2B6, or CYP2D6. Similarly, UGT1A1, UGT2B7, and aldehyde oxidase were not inhibited. Neither parent or ACP-5862 strongly induced CYP1A2, CYP2B6, or CYP3A4 mRNA. Acalabrutinib and ACP-5862 were substrates of MDR1 and BCRP in vitro, but were not substrates of OATP1B1 or OATP1B3. Acalabrutinib was not a substrate of OAT1, OAT3, and OCT2. Based on static PK model calculations, acalabrutinib may cause a modest increase in exposure to coadministered BCRP substrates by inhibition of intestinal BCRP, but with no inhibition of BCRP at the systemic level. The PK of substrates of MDR1, MATE1, MATE2-K, OATP1B1, OATP1B3, OAT1, OAT3, and OCT2 are not likely to be altered by acalabrutinib or ACP-5862. These data were combined with clinical DDI data (Izumi et al, 2017) to simulate DDI in the presence of CYP3A inhibitors and inducers. PBPK models confirmed that acalabrutinib and ACP-5862 were not likely to perpetrate CYP2C8 or CYP3A4 mediated drug interactions (Zhou et al., 2019). Overall, acalabrutinib and major metabolite, ACP-5862 have a favorable drug interaction profile.

Citation Format: Terry Podoll, Paul G. Pearson, Jerry Evarts, Tim Ingallinera, Hao Sun, Stephen Byard, Adrian J. Fretland, J. Greg Slatter. Structure elucidation, metabolism, and drug interaction potential of ACP-5862, an active, major, circulating metabolite of acalabrutinib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 13.

Bioavailability, Biotransformation, and Excretion of the Covalent Bruton Tyrosine Kinase Inhibitor Acalabrutinib in Rats, Dogs, and Humans

Acalabrutinib is a targeted, covalent inhibitor of Bruton tyrosine kinase (BTK) with a unique 2-butynamide warhead that has relatively lower reactivity than other marketed acrylamide covalent inhibitors. A human [¹⁴C] microtracer bioavailability study in healthy subjects revealed moderate intravenous clearance (39.4 l/h) and an absolute bioavailability of 25.3% ± 14.3% (n = 8). Absorption and elimination of acalabrutinib after a 100 mg [¹⁴C] microtracer acalabrutinib oral dose was rapid, with the maximum concentration reached in <1 hour and elimination half-life values of <2 hours. Low concentrations of radioactivity persisted longer in the blood cell fraction and a peripheral blood mononuclear cell subfraction (enriched in target BTK) relative to plasma. [¹⁴C]Acalabrutinib was metabolized to more than three dozen metabolites detectable by liquid chromatography-tandem mass spectrometry, with primary metabolism by CYP3A-mediated oxidation of the pyrrolidine ring, thiol conjugation of the butynamide warhead, and amide hydrolysis. A major active, circulating, pyrrolidine ring-opened metabolite, ACP-5862 (4-[8-amino-3-[4-(but-2-ynoylamino)butanoyl]imidazo[1,5-a]pyrazin-1-yl]-N-(2-pyridyl)benzamide), was produced by CYP3A oxidation.Novel enol thioethers from the 2-butynamide warhead arose from glutathione and/or cysteine Michael additions and were subject to hydrolysis to a β-ketoamide. Total radioactivity recovery was 95.7% ± 4.6% (n = 6), with 12.0% of dose in urine and 83.5% in feces. Excretion and metabolism characteristics were generally similar in rats and dogs. Acalabrutinib's highly selective, covalent mechanism of action, coupled with rapid absorption and elimination, enables high and sustained BTK target occupancy after twice-daily administration.

Syntheses of highly substituted enantiopure C6 and C7 enones(1)

Enantiopure epoxyvinyl sulfones SS-9a, SS-9b, produced from Jacobsen epoxidation of 2-phenylsulfonyl 1,3-cyclohexa- and cycloheptadiene, are used as a template for the construction of substituted cycloalkenones and as chiral synthetic equivalents of enones a and b. The addition of carbon nucleophiles to SS-9a, SS-9b is high yielding and stereospecific. Enantiopure alpha,beta- and gamma-substituted cycloalkenones are easily constructed using a variety of methods.

Cyclic AMP-binding proteins in human blood platelets detected by photoaffinity labelling

Cyclic AMP inhibits platelet aggregation induced by physiological agents. 8 Azido [32P]cyclic AMP (N3 cyclic AMP) has been utilized as a photoaffinity probe to define the cyclic AMP-binding proteins present in unperturbed human platelets and their subcellular fractions. Specificity of cyclic AMP binding was determined by contrasting binding in the presence and absence of excess unlabelled cyclic AMP, cyclic GMP and 5'-AMP. Binding was unaffected by 5'-AMP and obliterated by cyclic AMP. Four major species of binding proteins, 49 000, 42 000, 39 000, 37 000, were obtained in all platelet fractions (crude homeogenate, cytosol, membranes and granules). Two-dimensional gel electrophoresis of platelet cytosol resolved the major molecular weight species into 15 specific cyclic AMP binding proteins of four molecular weight classes differing by charge density. These studies suggest that platelets contain an array of specific cyclic AMP-binding proteins which may function in hemostatic regulation.