Paul McNamara

Exploratory drug safety: A discovery strategy to reduce attrition in development

Identification of novel new molecules which hold the greatest promise of safe and effective therapies remains a continuous challenge to the pharmaceutical industry. This has led the industry to implement strategies for identification of the most promising candidates during the discovery phase and for their safe and expeditious advancement through development. Testing for potential liable properties in the discovery phase has included the evaluation of major areas of pharmaceutics that have led to failure such as its physical and pharmaceutical properties, drug metabolism and pharmacokinetic characteristics, various safety endpoints including pre-development safety pharmacology, general toxicology and genetic toxicology and interrogation of counter-screen data to identify off-target affinities (i.e., receptors, ion channels, transporters, kinases, etc.) that pose a concern. Amongst the many important areas of concern is the potential for toxicities of the major organ systems. To mitigate this concern, a strategy pursued is to identify the prominent toxicological properties of the candidate prior to its recommendation for development. The results of these studies in discovery allow exclusion of the candidate before the expenditure of resources and time typical of development. In addition, the discovery phase toxicology studies serve to address key questions that may have arisen from the study of another molecule, the phenotypic profile from pre-clinical models where the therapeutic target has been genetically modified or concerns that have been raised as a result of other investigations. Importantly, the results of the exploratory drug safety studies will be used by the sponsor to judge the potential risks associated with continued pursuit of a potential development candidate. In many ways, pre-clinical toxicological investigations in discovery serve the important objective of identifying the most promising candidates to progress into development and onto registration.

Discovery of Narlaprevir (SCH 900518): A Potent, Second Generation HCV NS3 Serine Protease Inhibitor

Boceprevir (SCH 503034), 1, a novel HCV NS3 serine protease inhibitor discovered in our laboratories, is currently undergoing phase III clinical trials. Detailed investigations toward a second generation protease inhibitor culminated in the discovery of narlaprevir (SCH 900518), 37, with improved potency (∼10-fold over 1), pharmacokinetic profile and physicochemical characteristics, currently in phase II human trials. Exploration of synthetic sequence for preparation of 37 resulted in a route that required no silica gel purification for the entire synthesis.

Estimation of the Extent of Oral Absorption in Animals from Oral and Intravenous Pharmacokinetic Data in Drug Discovery

Oral administration is the most desirable route of drug delivery for systemically active drugs. Oral drugs must possess a certain level of oral bioavailability, which is a product of oral absorption and first-pass effect. Low oral bioavailability may be attributed to poor absorption and/or high first-pass hepatic elimination. In the lead optimization stage of drug discovery, if the relative contribution of oral absorption and metabolism could be discerned for poorly bioavailable compounds, a path forward for remedy would be possible. This report describes an approach utilizing oral/intravenous pharmacokinetic data to estimate oral absorption. The fraction of dose absorbed is calculated as the ratio of the actual bioavailable fraction to the maximum bioavailable fraction estimated from systemic clearance. An arbitrary classification was devised where low absorption encompasses compounds whose extent of absorption is <or=20%, moderate is for those between 21% and 69%, and high is for those that show >or=70% absorption. There was approximately 78% concordance in rats, 65% in monkeys and almost complete concordance in dogs. This approach correctly identified the cause for low oral bioavailability for 11 out of 13 compounds evaluated, and therefore it could be used prospectively with nonradiolabeled compounds during the lead optimization process.

The discovery of N-((2H-tetrazol-5-yl)methyl)-4-((R)-1-((5r,8R)-8-(tert-butyl)-3-(3,5-dichlorophenyl)-2-oxo-1,4-diazaspiro[4.5]dec-3-en-1-yl)-4,4-dimethylpentyl)benzamide (SCH 900822): a potent and selective glucagon receptor antagonist.

A novel series of spiroimidazolone-based antagonists of the human glucagon receptor (hGCGR) has been developed. Our efforts have led to compound 1, N-((2H-tetrazol-5-yl)methyl)-4-((R)-1-((5r,8R)-8-(tert-butyl)-3-(3,5-dichlorophenyl)-2-oxo-1,4-diazaspiro[4.5]dec-3-en-1-yl)-4,4-dimethylpentyl)benzamide (SCH 900822), a potent hGCGR antagonist with exceptional selectivity over the human glucagon-like peptide-1 receptor. Oral administration of 1 lowered 24 h nonfasting glucose levels in imprinting control region mice on a high fat diet with diet-induced obesity following single oral doses of 3 and 10 mg/kg. Furthermore, compound 1, when dosed orally, was found to decrease fasting blood glucose at 30 mg/kg in a streptozotocin-treated, diet-induced obesity mouse pharmacodynamic assay and blunt exogenous glucagon-stimulated glucose excursion in prediabetic mice.

Synthesis of 3H and 14C labelled SCH 48461

3 H-Sch 47949, racemic 3 H-Sch 48461, was prepared at a specific activity of 40 mCi/mmole by Pt catalysed exchange with tritiated water. 3 H-Sch 48461 was prepared at a specific activity of 64.6 Ci/mmole by a Pd/C catalysed reduction of an olefinic intermediate. 14 C-Sch 48461 was prepared in 8 steps from 14 C-potassium cyanide with an overall radiochemical yield of 18.5%.

Synthesis of 3H and 14C‐SCH 27899 by fermentation and evaluation of in vivo label stability

3H and 14 C-Sch 27899 have been prepared by fermentation using the Micromonospora carbonacea organism. In the case of 3 H-Sch 27899, the label was incorporated by a single addition of 100 mCi of 70Ci/mmole L-[3H-methyl]-methionine to two flasks. For 14 C-Sch 27899, 48 mCi of 55 mCi/mmole L-[14C-methyl]-methionine was added in five aliquots to five flasks over a five day period. Both batches were isolated by solvent extraction, oxidized and purified by column chromatography and hplc. An overall incorporation of 7.8% was found from L-[3H-methyl]-methionine and 18.7% from L-[14C-methyl]-methionine. The in vivo stability of label of 3 H and 14 C-Sch 27899 was determined, with 14 C-Sch 27899 found to be a better choice for use in in vivo metabolism studies.

Synthesis of 3H, 13C2, 2H4 14C-SCH 430765 and 35S-SCH 500946, potent and selective inhibitors of the NPY5 receptor

SCH 430765 and SCH 500496 are potent and selective antagonists of the NPY5 receptor. NPY5 receptor antagonists have the potential for the treatment of obesity. [35 S]SCH 500946 was prepared for a competition binding assay which led to the identification of SCH 430765. Three distinct isotopically labelled forms of SCH 430765 were synthesized. [3 H]SCH 430765 was prepared for a preliminary absorption, distribution, metabolism and excretion data evaluation of the compound and [14 C]SCH 430765 for more definitive absorption, distribution, metabolism and excretion data work. In addition, [13 C2 ,2 H4 ]SCH 430765 was prepared as an internal standard for a LC-MS bioanalytical method. The paper discusses the synthesis of 3 isotopically labelled forms of SCH 430765 and [35 S]SCH 500946.

SYNTHESIS OF (3) H, (2) H4 AND (14) C-MK 3814 (Preladenant).

MK 3814 is a potent and selective antagonist of the A2a receptor. A2a receptor antagonists have the potential for the treatment of Parkinson disease. Three distinct isotopically labelled forms of MK 3814 were synthesized. [3 H]MK 3814 was prepared for a preliminary absorption, distribution, metabolism, and excretion data (ADME) evaluation of the compound and [14 C]MK 3814 for more definitive ADME work, including an absorption, metabolism, and excretion study in man. In addition, [2 H4 ]MK 3814 was prepared as an internal standard for a liquid chromatography mass spectrometry bioanalytical method. This paper discusses the synthesis of 3 isotopically labelled forms of MK 3814.

Synthesis of3H,2H4, and14C-MK 3814 (preladenant)

MK 3814 is a potent and selective antagonist of the A2a receptor. A2a receptor antagonists have the potential for the treatment of Parkinson disease. Three distinct isotopically labelled forms of MK 3814 were synthesized. [3 H]MK 3814 was prepared for a preliminary absorption, distribution, metabolism, and excretion data (ADME) evaluation of the compound and [14 C]MK 3814 for more definitive ADME work, including an absorption, metabolism, and excretion study in man. In addition, [2 H4 ]MK 3814 was prepared as an internal standard for a liquid chromatography mass spectrometry bioanalytical method. This paper discusses the synthesis of 3 isotopically labelled forms of MK 3814.

Synthesis of (3) H, (2) H4 and (14) C-SCH 417690 (Vicriviroc).

Vicriviroc or SCH 417690 is a potent and selective antagonist of the CCR5 receptor. CCR5 receptor antagonists have the potential for the treatment of HIV infections. Four distinct isotopically labelled forms of SCH 417690 were synthesized. Low specific activity [(3) H]SCH 417690 was prepared for a preliminary absorption, distribution, metabolism and excretion evaluation of the compound and [(14) C]SCH 417690 for more definitive absorption, distribution, metabolism and excretion work, including an absorption, metabolism and excretion study in man. In addition, high specific activity [(3) H]SCH 417690 was prepared for CCR5 receptor binding work and [(2) H4 ]SCH 417690 was prepared as an internal standard for a liquid chromatography-mass spectrometry bioanalytical method. The paper discusses the synthesis of four isotopically labelled forms of SCH 417690.