Oral bioavailability screening

The importance of oral bioavailability screens early in the lead optimization process has been discussed in multiple articles and book chapters (Bohets et al., 2001 Chaturvedi et al., 2001 Mandagere et al., 2002 Korfmacher, 2003, 2009 ... 

Pintore et al., 2003 van de Waterbeemd and Jones, 2003 Crowley and Martini, 2004 Thompson, 2005 Avdeef et al., 2007 Cheng et al., 2007 Liu et al., 2008 Nomeir et al., 2009). Oral bioavailability can be viewed as a combination of two factors absorption and first-pass metabolism. Indeed, Cheng et al. (Lau et al., 2004 Cheng et al., 2006 Li et al., 2007) described a novel high-throughput in vitro assay based on a hybrid system that involved a combination of an absorption screen as well as metabolism screen in one system this system was used to evaluate multiple test compounds and has the potential to be used as a discovery oral bioavailability screening tool in the future. 

A standard model for higher-throughput oral screening is shown in Fig. 12.5. As shown in Fig. 12.5, a common scenario for an oral bioavailability screen would be to combine the results of an in vitro absorption screen with the results on an in vitro metabolic stability screen to select compounds to go into an in vivo oral PK screen. The NCEs that still appeared to be promising lead compounds after these various screens could then be selected for full PK studies (oral and intravenous dosing in three to four animals for each dose route). In this way, only those compounds that were likely to exhibit good oral bioavailability would be tested in the full PK studies. 

Very recently, the further screening of new benzy-loxyphenyl derivatives revealed a highly potent NCX inhibitor, named YM-244769. This orally bioavailable compound is more potent inhibiting NCX3 than NCX1 and NCX2 in the reverse mode, but it is not active on the forward mode of operation of the three antiporter isofoims. (Table 4)... 

Unity resulted in 4234 hits. After application of several filters and clustering of the remaining 1975 molecules, compounds from 18 of the 27 clusters were screened in Xenopus oocytes. One compound with an IC50 of 5.6pM belonged to a new class of Kvl.5 blockers and exhibited a favorable pharmacokinetic profile. After further optimization, compound 73 (IC50 = 0.7pM Fig. 16.9) resulted, with good oral bioavailability in rats [145]. 

Some drugs with low intrinsic permeability achieve acceptable oral bioavailability because they are substrates for uptake transporters, which normally function in nutrient uptake. The most prominent example is the peptide transporter, PepTl, which is active toward peptidomimetic antibiotics such as cephalexin, the antiviral agent valacyclovir [24] and other drugs. PepTl is natively expressed in Caco-2 cells, and adenovirus transduction has been used to increase PepTl expression levels [25]. However, the expression of PepTl was not polarized in this system and this expressed system appears to be of limited value as an improved screening model. PepTl has also been expressed in Chinese hamster ovary cells and a variety of other mammalian systems [26, 27]. 

The genesis of in silico oral bioavailability predictions can be traced back to Lip-inski s Rule of Five and others qualitative attempts to describe drug-like molecules [13-15]. These processes are useful primarily as a qualitative tool in the early stage library design and in the candidate selection. Despite its large number of falsepositive results, Lipinski s Rule of Five has come into wide use as a qualitative tool to help the chemist design bioavailable compounds. It was concluded that compounds are most likely to have poor absorption when the molecular weight is >500, the calculated octan-l-ol/water partition coefficient (c log P) is >5, the number of H-bond donors is >5, and the number of H-bond acceptors is >10. Computation of these properties is now available as an ADME (absorption, distribution, metabolism, excretion) screen in commercial software such as Tsar (from Accelrys). The rule-of-5 should be seen as a qualitative, rather than quantitative, predictor of absorption and permeability [16, 17]. 

This model uses in vitro data to estimate the oral bioavailability ranges of chemically diverse compounds in a range of species, and represents a potentially powerful tool when combined with high-throughput in vitro screening. 

Stoner CL, Cleton A, Johnson K, Oh D-M, Hallak H, Brodfuehrer J, Surendran N, Han H-K (2004) Integrated oral bioavailability projection using in vitro screening data as a selection tool in drug discovery. Int. J. Pharm. 269 241-249. 

LC/MS/MS techniques with selective and sensitive detection methods make it possible to quantitatively analyze samples from Caco-2 cell and PAMPA buffer matrices. A high-throughput permeability screen with robust LC/MS technology can quickly generate information about structure-permeability relationships that are extremely valuable in the lead optimization phase for the selection of pre-clinical candidates with favorable oral bioavailability properties. 

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