Elucidating the Mechanisms of Formation for Two Unusual Cytochrome P450-Mediated Fused Ring Metabolites of GDC-0623, a MAPK/ERK Kinase Inhibitor

Discovery of Unprecedented Human Stercobilin Conjugates

Two unique metabolites (M18 and M19) were detected in feces of human volunteers dosed orally with [14C]inavolisib with a molecular ion of parent plus 304 Da. They were generated in vitro by incubation with fecal homogenates and we have evidence that they are formed chemically and possibly enzymatically. Structural elucidation by high resolution mass spectrometry and nuclear magnetic resonance spectroscopy showed that the imidazole ring of inavolisib was covalently bound to partial structures derived from stercobilin, an end-product of heme catabolism produced by the gut microbiome. The structural difference between the two metabolites was the position of methyl and ethyl groups on the pyrrolidin-2-one moieties. We propose a mechanism of M18 and M19 generation from inavolisib and stercobilin whereby nucleophilic attack from the imidazole ring of inavolisib occurs to the bridging carbon of a stercobilin molecule. The proposed mechanism was supported by computational calculations of molecular orbitals and transition geometry. SIGNIFICANCE STATEMENT: We report the characterization of two previously undescribed conjugates of the phosphoinositide 3-kinase inhibitor inavolisib, generated by reaction with stercobilin, an end-product of heme catabolism produced by the gut microbiome. These conjugates were confirmed by generating them using in vitro fecal homogenate incubation via nonenzymatic and possibly enzymatic reactions. Given the unique nature of the conjugate, it is plausible that it may have been overlooked with other small molecule drugs in prior studies.

Mass Balance of the Indoleamine 2,3-Dioxygenase Inhibitor Navoximod (GDC-0919) in Rats and Dogs: Unexpected Cyanide Release from Imidazo[5,1-a]isoindole and Species Differences in Glucuronidation

Navoximod (GDC-0919) is a small molecule inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1) developed to reduce T cell immunosuppression associated with cancer. This study describes the absorption, metabolism, and excretion (AME) of navoximod in rats and dogs after a single oral dose of [14C]-navoximod. An unexpected thiocyanate metabolite M1 and a chiral inversion metabolite M51 were captured as the major circulating metabolites in rats, accounting for 30% and 18% of 0-24 hours exposure, respectively. These two metabolites combined had much lower systemic exposure in dogs and humans (<6% and <1%). The novel cyanide release is proposed to occur via 4,5-epoxidation on the fused imidazole ring, leading to ring opening and rearrangement along with the release of cyanide. The decyanated metabolites were identified and confirmed by synthetic standards, which supported the proposed mechanism. In dogs, glucuronidation to M19 was the major clearance mechanism, representing 59% of the dose in the bile of bile duct-cannulated (BDC) dogs and 19% of the dose in the urine of intact dogs. Additionally, M19 also represented 52% of drug related exposure in circulation in dogs. In comparison, in humans, navoximod was mainly cleared through glucuronidation to M28 and excreted in urine (60% of the dose). The differences in the metabolism and elimination observed in vivo were qualitatively recapitulated in vitro with liver microsomes, suspended hepatocytes, and cocultured primary hepatocytes. The striking species differences in regioselective glucuronidation is likely explained by the species differences in UGT1A9, which was mainly responsible for M28 formation in humans. SIGNIFICANCE STATEMENT: The results from this study demonstrated significant species differences in metabolism (especially glucuronidation) and elimination of navoximod among rats, dogs, and humans. The study also illustrated the mechanism of a novel cyanide release metabolism from the fused imidazo[5,1-a]isoindole ring. Such biotransformation should be kept in mind when working with imidazole-containing new chemical entities in drug discovery and development.

MEK inhibitors for the treatment of non-small cell lung cancer

BRAF and KRAS are two key oncogenes in the RAS/RAF/MEK/MAPK signaling pathway. Concomitant mutations in both KRAS and BRAF genes have been identified in non-small cell lung cancer (NSCLC). They lead to the proliferation, differentiation, and apoptosis of tumor cells by activating the RAS/RAF/MEK/ERK signaling pathway. To date, agents that target RAS/RAF/MEK/ERK signaling pathway have been investigated in NSCLC patients harboring BRAF mutations. BRAF and MEK inhibitors have gained approval for the treatment of patients with NSCLC. According to the reported findings, the combination of MEK inhibitors with chemotherapy, immune checkpoint inhibitors, epidermal growth factor receptor-tyrosine kinase inhibitors or BRAF inhibitors is highly significant for improving clinical efficacy and causing delay in the occurrence of drug resistance. This review summarized the existing experimental results and presented ongoing clinical studies as well. However, further researches need to be conducted to indicate how we can combine other drugs with MEK inhibitors to significantly increase therapeutic effects on patients with lung cancer.

Metabolism and Disposition of a Novel B-Cell Lymphoma-2 Inhibitor Venetoclax in Humans and Characterization of Its Unusual Metabolites

Venetoclax (ABT-199), a B-cell lymphoma-2 (Bcl-2) protein inhibitor, is currently in clinical development for the treatment of hematologic malignancies. We characterized the absorption, metabolism, and excretion of venetoclax in humans. After a single oral dose of [14C]venetoclax to healthy volunteers, the recovery of total radioactive dose was 100%, with feces being the major route of elimination of the administered dose, whereas urinary excretion was minimal (<0.1%). The extent of absorption was estimated to be at least 65%. Venetoclax was primarily cleared by hepatic metabolism (∼66% of the administered dose). ∼33% of the administered dose was recovered as the parent drug and its nitro reduction metabolite M30 [2-((1H-pyrrolo[2,3-b]pyridin-5-yl)oxy)-N-((3-amino-4-(((tetrahydro-2H-pyran-4-yl)methyl)amino)phenyl)sulfonyl)-4-(4-((4'-chloro-5,5-dimethyl-3,4,5,6-tetrahydro-[1,1'-biphenyl]-2-yl)methyl)piperazin-1-yl)benzamide] (13%) in feces. Biotransformation of venetoclax in humans primarily involves enzymatic oxidation on the dimethyl cyclohexenyl moiety, followed by sulfation and/or nitro reduction. Nitro reduction metabolites were likely formed by gut bacteria. Unchanged venetoclax was the major drug-related material in circulation, representing 72.8% of total plasma radioactivity. M27 (oxidation at the 6 position of cyclohexenyl ring followed by cyclization at the α-carbon of piperazine ring; 4-[(10aR,11aS)-7-(4-chlorophenyl)-9,9-dimethyl-1,3,4,6,8,10,10a,11a-octahydropyrazino[2,1-b][1,3]benzoxazin-2-yl]-N-[3-nitro-4-(tetrahydropyran-4-ylmethylamino)phenyl]sulfonyl-2-(1H-pyrrolo[2,3-b]pyridin-5-yloxy)benzamide) was identified as a major metabolite, representing 12% of total drug-related material. M27 was primarily formed by cytochrome P450 isoform 3A4 (CYP3A4). Steady-state plasma concentrations of M27 in human and preclinical species used for safety testing suggested that M27 is a disproportionate human metabolite. M27 is not expected to have clinically relevant on- or off-target pharmacologic activities.