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

Abstract

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 following a single oral dose of [ 14 C]-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 hr 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 co-cultured 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.

The Importance of Tracking “Missing” Metabolites: How and Why?

“…However, a novel cyanide release metabolism from the fused imidazo[5,1-a]isoindole ring of navoximod was observed in rat, dog, and human (Figure 2B). 7,27 The released radiolabeled cyanide was thiolated to thiocyanate by Rhodanese, as mentioned earlier, which has a long elimination half-life, contributing to the apparent long-lived radioactivity in circulation. The thiocyanate metabolite could be easily missed without the radiolabel at the C2-imidazole position.…”

“…For example, the toxic cyanide metabolite can be released from alkyl nitriles and sometimes even imidazoisoindoles. 7,8 It was also once thought in medicinal chemistry drug design that decyanation metabolism might not occur with aromatic−nitrile groups; 9 however, instances of decyanation from heteroaromatic moieties have been reported (Figure 1A). 10,11 In humans, cyanide can be efficiently detoxified via the enzyme Rhodanese to form thiocyanate but is indistinguishable from endogenous thiocyanate.…”

“…However, a small metabolite invisible in LC-MS can sometimes be under scrutiny. For example, the toxic cyanide metabolite can be released from alkyl nitriles and sometimes even imidazo­isoindoles. , It was also once thought in medicinal chemistry drug design that decyanation metabolism might not occur with aromatic–nitrile groups; however, instances of decyanation from heteroaromatic moieties have been reported (Figure A). , In humans, cyanide can be efficiently detoxified via the enzyme Rhodanese to form thiocyanate but is indistinguishable from endogenous thiocyanate . Radiolabeling nitrile groups with 14 C can be the most direct method of revealing drug-related cyanide release and subsequent formation of the detoxified thiocyanate metabolite.…”