Substrate orientation and the origin of catalytic power in xanthine oxidoreductase

355-362With the chemical course of the reaction catalyzed by the molybdenum-containing hydroxylase xanthine oxidoreductase now relatively well-understood, efforts in the field have now turned to understanding the catalytic power of the enzyme in the context of its structure. The present minireview is an account of recent efforts, from the authors’ laboratory and elsewhere, towards understanding the role of active site amino acid residues in accelerating reaction rate. On the basis of recent site-directed mutagenesis work, in conjunction with protein X-ray crystallography, it is now possible to attribute the specific extent to which each contributes to transition state stabilization and the means by which this occurs

Substrate Orientation and Catalysis at the Molybdenum Site in Xanthine Oxidase: CRYSTAL STRUCTURES IN COMPLEX WITH XANTHINE AND LUMAZINE*

Xanthine oxidoreductase is a ubiquitous cytoplasmic protein that catalyzes the final two steps in purine catabolism. We have previously investigated the catalytic mechanism of the enzyme by rapid reaction kinetics and x-ray crystallography using the poor substrate 2-hydroxy-6-methylpurine, focusing our attention on the orientation of substrate in the active site and the role of Arg-880 in catalysis. Here we report additional crystal structures of as-isolated, functional xanthine oxidase in the course of reaction with the pterin substrate lumazine at 2.2 Å resolution and of the nonfunctional desulfo form of the enzyme in complex with xanthine at 2.6 Å resolution. In both cases the orientation of substrate is such that the pyrimidine subnucleus is oriented opposite to that seen with the slow substrate 2-hydroxy-6-methylpurine. The mechanistic implications as to how the ensemble of active site functional groups in the active site work to accelerate reaction rate are discussed

A First-in-Human Phase 1 Study of Oral LOXO-338, a Selective BCL2 Inhibitor, in Patients with Advanced Hematologic Malignancies (Trial in Progress)

Abstract Background: B-cell lymphoma 2 (BCL2) is a key regulator of apoptosis and provides protection from cell death in many hematological malignancies. The BCL2 inhibitor venetoclax is approved for the treatment of CLL/SLL and acute myeloid leukemia and has activity in other lymphoid malignancies. LOXO-338 is a novel, orally bioavailable small molecule inhibitor of BCL2, designed to achieve selectivity over BCL-xL and thus avoid dose-limiting thrombocytopenia associated with BCL-xL inhibition. In preclinical studies, LOXO-338 showed a favorable pharmacological profile, selectively inhibited BCL2, and was well-tolerated in vivo. LOXO-338 also demonstrated dose-dependent tumor growth inhibition in various murine xenograft models, and showed improved efficacy in combination with pirtobrutinib, a highly selective, non-covalent BTK inhibitor (Brandhuber et al. Cancer Res 2021; 81, 13 Supplement, 1258). Study Design and Methods: LOXO-BCL-20001 is an open-label, multi-center, first-in-human Phase 1 study of oral LOXO-338 in patients with advanced hematologic malignancies who have received standard therapy. The study will be conducted in 2 parts. Part 1 will evaluate LOXO-338 as monotherapy, and will explore different dosing strategies. Part 2 will evaluate LOXO-338 in combination with pirtobrutinib. The dose escalation portion of the study in Part 1 will follow an i3+3 design. Each cycle will be 28 days (4 weeks). Eligible patients include those with CLL/SLL, mantle cell lymphoma (MCL), and Waldenstrӧm macroglobulinemia (WM) who have already received standard therapy. Patients with other B-cell non-Hodgkin lymphomas (NHLs) who failed standard therapy or, in the opinion of the investigator, have no known available options to provide benefit for the patient's condition, are also eligible. Patients must have recovered from prior treatment-related adverse events. Patients with active or suspected Richter transformation, transformed low grade lymphoma, Burkitt or Burkitt-like lymphoma, and multiple myeloma (MM) are eligible in dose-expansion. Key exclusion criteria include history of CNS involvement, stem cell transplant or CAR-T therapy <60 days, concurrent anticancer therapy, and clinically significant cardiovascular disease. The primary objective of Part 1 is to determine the maximum tolerated dose (MTD)/ recommended Phase 2 dose (RP2D) of oral LOXO-338 in patients who were previously treated for CLL/SLL and other B-cell NHLs. Key secondary objectives include determining the safety and tolerability, and pharmacokinetic properties of LOXO-338. Antitumor activity will be evaluated based on overall response rate (ORR), progression-free survival (PFS), time to progression (TTP) and duration of response (DOR) based on disease-specific response criteria per investigator assessment. Key objectives of part 2 are to determine the safety profile and tolerability, PK properties, and anti-tumor activity of LOXO-338 in combination with pirtobrutinib. Disclosures Alencar: Seattle Genetics: Consultancy; Kite Pharma: Consultancy; Karyopharm: Consultancy; Janssen: Consultancy; Incyte: Consultancy; Epizyme: Consultancy; Celgene: Consultancy; BeiGene: Consultancy; Amgen: Consultancy. Roeker: AbbVie, AstraZeneca, Janssen, LOXO, Pharmacyclics, TG Therapeutics, Vaniam Group, Verastem: Consultancy; Pharmacyclics: Consultancy; Pfizer: Consultancy, Research Funding; Loxo Oncology: Consultancy; TG Therapeutics: Consultancy; Abbot Laboratories: Current equity holder in publicly-traded company. Hoffmann: TG Therapeutics: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Pharmcyclics: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; celgene: Consultancy, Honoraria. Guru Murthy: Cancerexpertnow: Honoraria; Guidepoint: Consultancy; Techspert: Consultancy; Qessential: Consultancy; Cardinal Health Inc.: Honoraria; TG therapeutics: Other: Advisory board. Patel: Loxo Oncology at Lilly: Current Employment, Current equity holder in publicly-traded company. Ku: Loxo Oncology at Lilly: Current Employment, Current holder of stock options in a privately-held company. Pauff: Loxo Oncology at Lilly: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company. Eyre: Incyte: Consultancy; Loxo Oncology: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Beigene: Honoraria, Research Funding; Roche: Consultancy, Honoraria; Gilead/KITE: Honoraria, Other: Travel support for conferences, Research Funding, Speakers Bureau; Secura Bio: Consultancy, Honoraria; Janssen: Honoraria; Abbvie: Consultancy, Honoraria, Other: Travel to conferences; AstraZeneca: Honoraria, Research Funding. Jurczak: Celtrion: Research Funding; Celgene: Research Funding; Debbiopharm: Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding; Loxo Oncology: Membership on an entity's Board of Directors or advisory committees; Sandoz: Membership on an entity's Board of Directors or advisory committees; BeiGene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer: Research Funding; Astra Zeneca: Membership on an entity's Board of Directors or advisory committees, Research Funding; Abbvie: Research Funding; Roche: Membership on an entity's Board of Directors or advisory committees, Research Funding; Epizyme: Research Funding; Incyte: Research Funding; Merck: Research Funding; Takeda: Research Funding; TG Therapeutics: Research Funding. OffLabel Disclosure: LOXO-338 is a novel, orally bioavailable small molecule inhibitor of BCL2 for advanced hematologic malignancies

Mechanism of Substrate and Inhibitor Binding of Rhodobacter capsulatus Xanthine Dehydrogenase*

Rhodobacter capsulatus xanthine dehydrogenase (XDH) is an (αβ)2 heterotetrameric cytoplasmic enzyme that resembles eukaryotic xanthine oxidoreductases in respect to both amino acid sequence and structural fold. To obtain a detailed understanding of the mechanism of substrate and inhibitor binding at the active site, we solved crystal structures of R. capsulatus XDH in the presence of its substrates hypoxanthine, xanthine, and the inhibitor pterin-6-aldehyde using either the inactive desulfo form of the enzyme or an active site mutant (EB232Q) to prevent substrate turnover. The hypoxanthine- and xanthine-bound structures reveal the orientation of both substrates at the active site and show the importance of residue GluB-232 for substrate positioning. The oxygen atom at the C-6 position of both substrates is oriented toward ArgB-310 in the active site. Thus the substrates bind in an orientation opposite to the one seen in the structure of the reduced enzyme with the inhibitor oxypurinol. The tightness of the substrates in the active site suggests that the intermediate products must exit the binding pocket to allow first the attack of the C-2, followed by oxidation of the C-8 atom to form the final product uric acid. Structural studies of pterin-6-aldehyde, a potent inhibitor of R. capsulatus XDH, contribute further to the understanding of the relative positioning of inhibitors and substrates in the binding pocket. Steady state kinetics reveal a competitive inhibition pattern with a Ki of 103.57 ± 18.96 nm for pterin-6-aldehyde