Stability microsome

Stndies of the antoxidation of carotenoids in liposomal suspensions have also been performed since liposomes can mimic the environment of carotenoids in vivo. Kim et al. stndied the antoxidation of lycopene," P-carotene," and phytofluene" " in liposomal snspensions and identified oxidative cleavage compounds. Stabilities to oxidation at room temperature of various carotenoids incorporated in pig liver microsomes have also been studied." The model took into account membrane dynamics. After 3 hr of reactions, P-carotene and lycopene had completely degraded, whereas xanthophylls tested were shown to be more stable. 

Socaciu, C., Jessel, R., and Diehl, H.A., Carotenoid incorporation into microsomes yields, stability and membrane dynamics, Spectrochim. Acta A Mol. Biomol. Sped, 56, 2799, 2000. 

A wide range of groups are tolerated in this position including esters, amides, thioesters, ureas, acyl hydrazines and carbamates exhibiting a wide range of potencies and microsomal stabilities (Table 6.2) [24, 33-36]. 

With this focus on CYP and fiver metabolism, most companies have established high throughput assays to measure compound stability in the presence of human (or preclinical species) fiver microsomes [49]. Disappearance of starting compound from an incubation with microsomes is monitored. Measurement at a single time point enables a rank-ordering of compounds for stability based on percent of parent compound remaining acquisition of data at multiple time points allows determination of half-life, intrinsic clearance, and extrapolation to a predicted in vivo clearance [50]. 

A lead is variously defined in the pharmaceutical industry as a compound derived from a hit with some degree of in vitro optimization (potency in primary assay, activity in functional and/or cellular assay), optimization of physical properties (solubility, permeability), and optimization of in vitro ADME properties (microsomal stability, CYP inhibition). Moreover, a lead must have established SAR/SPR around these parameters such that continued optimization appears possible. A lead may also have preliminary PK and in vivo animal model data. However, it is the task of the lead optimization chemist to improve PK and in vivo activity to the levels needed for identification of a clinical candidate. 

As mentioned in Sect. 3, it is important to establish a detailed lead profile at the beginning of a lead identification effort. Criteria vary in different lead identification or hit-to-lead groups, but generally include some or all of the following potency, functional activity, selectivity, MW, clogP, solubility, permeability, microsomal stability and/or hepatocyte clearance, and preliminary PK including oral bioavail-ability. An example of a lead profile for a kinase inhibitor project is illustrated in Table 1 [21],... 

IkB kinase-p is a key regulatory enzyme in the NF-kB pathway, and inhibition of this enzyme has the potential for yielding treatments for inflammatory and autoimmune diseases. Morwick et al. [53] report on the optimization of a pM IKKp inhibitor with low aqueous solubility, moderate human liver microsome stability, and inhibition of several CYPs (3A4, 2C9, 1A2) with pM potencies. Modulation of the thiophene core (other thiophene isomer, pyrimidine and oxazole) produces compounds of similar potency to the hit. Fusing the 5-phenyl moiety to the thiophene to form a thieno[2,3-b]pyridine core increases aqueous solubility of the series as well as reduces the CYP liability. While the optimized compound still shows pM IKK(S potency, the aqueous solubility, HLM stability and CYP profiles are much improved. A pharmacophore model was generated that enabled scaffold hopping to yield this new chemotype (Scheme 7). 

Forti, G.C., Paolini, M., Hrelia, P. et al. (1984) NADPH-generating system influence on microsomal monooxygenase stability during incubation for the liver microsomal assay with rat and mouse S9 fractions. Mutation Research, 129, 291-297. 

The aim of the present example was to investigate whether the assessment of an in silico model of metabolic stability from a training set of several hundred drugs or drug-like compounds in human CYP3A4 cDNA-expressed microsomal preparations, would offer a suitable approach to predict the metabolic stability of external compounds. 

This model integrates existing in vitro data, such as Caco-2 permeability (Papp) and metabolic stability in liver S9 or microsomes, to estimate bioavailability as being either low, medium, or high. Oral bioavailability predictions for not only humans but also other species can be made by using the metabolic stability values of drugs in liver microsomal enzyme preparations from that species. A premise of this model is that metabolic clearance is more important than renal or biliary clearance in determining bioavailability. However, despite the lack of in vitro renal... 

Fig. 19.10. Oral bioavailability estimates of drugs in rat, guinea pig, dog and human from their respective Caco-2 Papp and metabolic stability in liver microsomes or S9. (Reproduced with permission from Ref. [26], 2002 American Chemical Society.)...

Compound 24 represents a series of potent spiropiperidine GlyT-1 inhibitors that showed significant binding to the p opioid and nociceptin/ orphanin FQ peptide (NOP) receptors [67]. The hydroxy analog 25 demonstrated improved selectivity and N-propyl analog 26 exhibited increased microsomal stability but reduced GlyT-1 potency (EC50 = 450 nM) [68]. 

Naphthyl inhibitor 38 (Kj = 500 nM) was discovered via a displacement-based HTS assay utilizing radiolabeled (R)-NPTS [77]. However, compound 38 demonstrated high affinity for 5-HT1B (Kj < 1 nM), hERG (K, = 1.2 iM), significant inhibition of CYP 2D6, and poor microsomal stability. 

Modification of 38 improved the selectivity versus 5-HTiB however, hERG activity, CYP 2D6 inhibition, and poor microsomal stability remained. The series was discontinued due to a lack of progress with overcoming these challenges [77]. 

An important DMPK property of a NCE is oral bioavailability (F) of the compound in various pre-clinical species.3 The oral bioavailability of a compound is dependent on several factors including intestinal permeability (estimated by the Caco-2 assay) and hepatic clearance (estimated with an in vitro metabolic stability assay).3 30 The metabolic stability assay is typically performed by incubating test compounds in liver microsomes or hepatocytes. The results can provide estimates of in vivo stability in terms of metabolic liabilities.3 8 59 62 Several authors described this assay as an important tool for the rapid assessment of the DMPK properties of NCEs.3 6 8111819 26 44 59 62-65... 

Some authors searched for common oxidative metabolites as part of metabolic stability assays. Tong et al.75 described a highly automated microsomal metabolic stability assay that achieved a throughput of 50 compounds per day with each compound tested in rats, dogs, monkeys, and humans. In addition to assaying the test compound, they monitored M+16 metabolites by using the... 

Advances in techniques for chemical synthesis allow medicinal chemists to synthesize hundreds to thousands of compounds per month. Metabolic stability screening in liver microsomes is used extensively in early discovery to select the analogs or compounds most likely to have favorable pharmacokinetic parameters. This provides information on the relation of structure to stability, thus guiding synthesis strategies. 

Korfmacher, W. A. et al. 1999. Development of an automated mass spectrometry system for the quantitative analysis of liver microsomal incubation samples A tool for rapid screening of new compounds for metabolic stability. Rapid Commun. Mass Spectrom. 13 901. 

Xu, R. et al. 2002. Application of parallel liquid chromatography/mass spectrometry for high throughput microsomal stability screening of compound libraries. J. Am. Soc. Mass Spectrom. 13 155. 

Ashwood-Smith, M.J. (1980). Stability of frozen microsome preparations for use in the Ames Salmonella mutagenicity assay. Mutation Res. 69 199-200. 

Several kinetic parameters can be measured on different experimental systems to account for the interaction of a compound with CYPs. For example when studying the metabolic stability of a compound, it could be measured in a recombinant CYP system, in human liver microsomes, in hepatocytes and so on. Each system increases in biological complexity. Although in the recombinant CYP system only the cytochrome under consideration is studied, in the case of the human liver microsomes, there is a pool of enzyme present that includes several CYPs, and finally in the hepatocyte cell system, metabolizing enzymes play an important role in the metabolic compound stability. In addition, transport systems are also present that could involve recirculation or other transport phenomena. The more complex the experimental system, the more difficult it is to extract information on the protein/ligand interaction, albeit it is closer to the in vivo real situation and therefore to the mechanism that is actually working in the body. 

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