Characterization of Divergent Metabolic Pathways in Elucidating an Unexpected, Slow-Forming, and Long Half-Life Major Metabolite of Iclepertin.

PURPOSE: After single oral dosing of the glycine reuptake transporter (GlyT1) inhibitor, iclepertin (BI 425809), a single major circulating metabolite, M530a, was identified. However, upon multiple dosing, a second major metabolite, M232, was observed with exposure levels ~ twofold higher than M530a. Studies were conducted to characterize the metabolic pathways and enzymes responsible for formation of both major human metabolites. METHODS: In vitro studies were conducted with human and recombinant enzyme sources and enzyme-selective inhibitors. The production of iclepertin metabolites was monitored by LC-MS/MS. RESULTS: Iclepertin undergoes rapid oxidation to a putative carbinolamide that spontaneously opens to an aldehyde, M528, which then undergoes reduction by carbonyl reductase to the primary alcohol, M530a. However, the carbinolamide can also undergo a much slower oxidation by CYP3A to form an unstable imide metabolite, M526, that is subsequently hydrolyzed by a plasma amidase to form M232. This difference in rate of metabolism of the carbinolamine explains why high levels of the M232 metabolite were not observed in vitro and in single dose studies in humans, but were observed in longer-term multiple dose studies. CONCLUSIONS: The long half-life iclepertin metabolite M232 is formed from a common carbinolamine intermediate, that is also a precursor of M530a. However, the formation of M232 occurs much more slowly, likely contributing to its extensive exposure in vivo. These results highlight the need to employ adequate clinical study sampling periods and rigorous characterization of unexpected metabolites, especially when such metabolites are categorized as major, thus requiring safety assessment.

A unique sequential C-S or N-S inductive cleavage and retro-Diels-Alder fragmentation mechanism for alkylsulfonyl piperidine- and piperazine-containing compounds.

RATIONALE: BI 605906 undergoes a collision-induced dissociation (CID) fragmentation resulting in the loss of methylsulfinic acid and butadiene to produce a corresponding imine. The fragmentation is hypothesized to occur via inductive cleavage of the C-S bond, generating a six-membered cyclic ene, followed by the retro-Diels-Alder (RDA) reaction. The aim of this study was to provide mechanistic evidence for the proposed fragmentation by investigating the CID spectra of BI 605906 and other alkylsulfonyl piperidine- and piperazine-containing compounds. METHODS: The positive electrospray ionization tandem mass spectrometric (ESI+ -MS/MS) fragmentations of BI 605906, D9 -BI 605906, GK02935, GK02942, ketoconazole, terazosin, and homopiperazine were investigated. Additionally, incubations of BI 605906 and GK02942 in human liver microsomes (HLM) preparations were conducted. Metabolite identification experiments were performed following these incubations to investigate corresponding in vitro metabolism. RESULTS: BI 605906, D9 -BI 605906, GK02935, and GK02942 demonstrated the same fragmentation pattern by generating a respective imine ion, supporting the hypothesized inductive cleavage and subsequent RDA mechanism. Ketoconazole and terazosin, which contain either an N-acetyl or tetrahydrofuranyl piperazine group, respectively, did not demonstrate this mechanism, notably because they do not have the alkylsulfonyl moiety as a good leaving group. Although homopiperazine contains an arylsulfonyl diazepane group, and the initial step produced an unsaturated diazepane ring, the subsequent RDA reaction was unable to proceed due to the absence of a six-membered cyclic ene intermediate. Additionally, we identified oxidative metabolites of BI 605906 and GK02942 in HLM incubations utilizing the proposed fragmentation pattern. CONCLUSIONS: In the mass spectrometer, compounds containing alkylsulfonyl piperidine or piperazine groups can undergo inductive cleavage, leading to a six-membered cyclic ene intermediate. This intermediate will then form a corresponding imine ion via the RDA reaction. A practical application of this work is to utilize this fragmentation for elucidating structures of metabolites arising from parent compounds containing alkylsulfonyl piperidine or piperazine moieties.

N-Methylation of BI 187004 by Thiol S-Methyltransferase.

BI 187004, an 11ß-hydroxysteroid dehydrogenase 1 inhibitor, was administered once daily for 14 days to eight patients with type 2 diabetes mellitus. N-methylation was identified as a major biotransformation pathway. In four patients treated with BI 187004, the plasma exposure of an N-methylbenzimidazole metabolite [N-methylbenzimidazole regioisomer 1 (M1)] was 7-fold higher than the remaining four patients, indicating a substantial degree of metabolic variation. To identify the methyltransferase enzymes responsible for N-methylation, BI 187004 was incubated with human liver microsomes (HLM), human kidney microsomes (HKM), and their respective cytosolic preparations in the presence and absence of isoform-selective chemical inhibitors. Additionally, BI 187004 was incubated with several human recombinant methyltransferases: catechol O-methyltransferase (rhCOMT), histamine N-methyltransferase (rhHNMT), nicotinamide N-methyltransferase (rhNNMT), glycine N-methyltransferase (rhGNMT), and thiopurine S-methyltransferase (rhTPMT). M1 was principally observed in HLM and HKM incubations, minimally formed in liver and kidney cytosol, and not formed during incubations with recombinant methyltransferase enzymes. In all microsomal and cytosolic incubations, the formation of M1 was inhibited only by 2,3-dichloro-α-methylbenzylamine (DCMB), an inhibitor of thiol S-methyltransferase (TMT), providing evidence that TMT catalyzed the formation of M1. Interestingly, the N-methylbenzimidazole regioisomer (M14) was only observed in vitro, predominantly during incubations with human kidney cytosol and rhHNMT. The formation of M14 was inhibited by amodiaquine (an HNMT inhibitor) and DCMB, providing additional evidence that both HNMT and TMT catalyzed M14 formation. Overall, using BI 187004 as a substrate, this study demonstrates a novel TMT-mediated N-methylation biotransformation and an HNMT-mediated regioselective N-methylation.

Synthesis and characterization of constrained peptidomimetic dipeptidyl peptidase IV inhibitors: amino-lactam boroalanines.

We describe here the epimerization-free synthesis and characterization of a new class of conformationally constrained lactam aminoboronic acid inhibitors of dipeptidyl peptidase IV (DPP IV; E.C. 3.4.14.5). These compounds have the advantage that they cannot undergo the pH-dependent cyclization prevalent in most dipeptidyl boronic acids that attenuates their potency at physiological pH. For example, D-3-amino-1-[L-1-boronic-ethyl]-pyrrolidine-2-one (amino-D-lactam-L-boroAla), one of the best lactam inhibitors of DPP IV, is several orders of magnitude less potent than L-Ala-L-boroPro, as measured by Ki values (2.3 nM vs 30 pM, respectively). At physiological pH, however, it is actually more potent than L-Ala-L-boroPro, as measured by IC50 values (4.2 nM vs 1400 nM), owing to the absence of the potency-attenuating cyclization. In an interesting and at first sight surprising reversal of the relationship between stereochemistry and potency observed with the conformationally unrestrained Xaa-boroPro class of inhibitors, the L-L diastereomers of the lactams are orders of magnitude less effective than the D-L lactams. However, this interesting reversal and the unexpected potency of the D-L lactams as DPP IV inhibitors can be understood in structural terms, which is explained and discussed here.

Synthesis and structural investigation of internally coordinated alpha-amidoboronic acids.

[structure: see text] Six new N-acyl-boroGly derivatives, along with their N-acyl-boroSar analogues, have been synthesized by modification of conventional procedures. Structural characterization of these alpha-amidoboronic acids was accomplished by extensive use of 11B and 1H NMR spectroscopy. These compounds were prepared to determine the extent of intramolecular B-O dative bond formation within the context of a five-membered (:O=C-N-C-B) ring motif. It is shown that the formation of such dative bonds depends on the nature of the substituents at both the acyl carbon and the nitrogen atoms. Computational evidence from second-order Møller-Plesset perturbation theory is provided in support of these findings.

Autochelation in dipeptide boronic acids: pH-dependent structures and equilibria of Asp-boroPro and His-boroPro by NMR spectroscopy.

Many dipeptide boronic acids of the type H(2)N-X-Y-B(OH)(2) are potent protease inhibitors. Interest in these compounds as drugs for cancer, diabetes, and other diseases is growing. Because of the great mutual B-N affinity, cyclization through the N- and B-termini, forming six-membered rings, is a common occurrence at neutral pH and higher where the terminal amino group is unprotonated. Here we report the discovery that when X, the N-terminal amino acid, contains a side chain having a functional group with boron affinity and suitable geometry, additional cyclization in the form of bidentate intramolecular chelation or "autochelation" may occur, predominantly at mid pH. NMR studies of two compounds, l-Aspartyl-l-boroProline (Asp-boroPro) and l-Histidyl-l-boroProline (His-boroPro), are reported here from pH 0.5 to pH 12 by (1)H, (15)N, (13)C, and (11)B NMR. Both of these previously unreported autochelates contain two fused six-membered rings, cis-proline, chiral boron, and -NH(2)(+) protons in slow exchange with water, even at 25 degrees C and pH as high as 4. Using microscopic acid-base equilibrium constants, we show that at high pH (>8 for Asp-boroPro and >10 for His-boroPro) hydroxide competes with the side chains for boron, reducing the chelates from bidentate to monodentate. At low pH (<0.5), proton competition for N-terminal nitrogens causes both compounds to become noncyclic. High chelate stability causes a reduction of the apparent acidic dissociation constant of the protonated N-terminal amino group greater than eight units. In the His-boroPro autochelate, imidazolate anion is produced at the extraordinarily low pH value of approximately 9.