P-gp Structures

TM helix 1 as well as the long linker joining NBD 1 with TMD2. The slice through the 

NBD region (bottom right panel) also shows a good overall fit between the map and 

The overall impression from this comparison is that Savl866 probably represents a reasonably good starting model for P-gp in the presence of AMP-PNP. There are differences in the precise trajectories of transmembrane helical elements, but this is to be expected given the low sequence identity between the two proteins in the transmembrane region ( 20%). A model of P-gp based on Savl866 but using adjustments or constraints from the EM-derived map would therefore be informative. 

The involvement of P-gp in the multidrug resistance phenotype has prompted extensive study to uncover the mechanism of this polyspccificity. Two possibilities are most likely (i) the presence of a single domain to which drugs display a loose association and (ii) multiple drug binding sites, each with a defined specificity. 

The quest to determine whether P-gp contains multiple drug binding sites employed a number of distinct approaches. For example, site-directed mutagenesis revealed a multitude of residues that when mutated could alter the pattern of drug resistance conferred by P-gp [183-185]. The effects of mutations on the activity were 

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The investigation of the drug-binding open-inside conformation of P-gp by MD simulation still faces numerous problems, due to instability of the mouse P-gp structure. In that sense the validity of this model is somewhat doubted (Gyimesi et al., 2011 Loo et al., 2010). 

Validation studies in manufacturing businesses and discussion with experts led to the view that knowledge used to define p, gp, tp and. Vp, could be structured such that may be formulated as ... 

Most ABC-transporters, especially those located in the plasma membrane, are phosphorylated and glycosylated transmembrane proteins of different molecular weights (e.g., P-gp 170 kDa MRP2 190 kDa BCRP 72 kDa). Topologically, most ABC-transporter show a similar structure they are organized in two transmembrane domains (TMD), each consisting of six... 

The multidrug resistance (MDR) phenomenon has first been described in cancer cells which were resistant against several structurally unrelated anticancer dtugs. Mechanistically a high expression of different ABC-transporters (P-gp, MRP) has been identified as the underlying factor. 

The structures of the drugs used as a small test set for the model are listed in Table 17.1. Loperamide and asimadoline are P-gp substrates terfenadine and ebastine are compounds that are rapidly metabolized alprazolam, dobazam, di-and mono-hydroxy-L66858 [11] are benzodiazepines testosterone and corticosterone are hormones and cefadroxyl, cefaclor, cephalotin and cefmetazole are cephalosporins [12]. Finally, peptides 1 to 10 are peptidomimetic drugs [13]. 

A rational approach to problems such as drug targeting to regions protected by P-gp, inhibition or even prevention of multidrug resistance in different organisms, and prediction of drug-drug interactions due to P-gp should become possible with reliable structure-activity relationship (SAR) studies. 

A comprehensive list of P-gp modulators or inhibitors, classified according to their chemical structures, has been published recently [87]. This shows that the structures of inhibitors are almost as heterogeneous as those of the substrates. A small but representative selection of inhibitors is shown in Fig. 20.12 and Table 20.1. In an attempt to clarify the different mechanisms of P-gp modulation or inhibition, the H-bonding concept discussed above is applied. To this end, the modulators or inhibitors in Table 20.1 were ordered according to their H-bond acceptor potential and divided in three groups comprising compounds with (i) a low EUh (<2 i.e., not transported) (ii) an intermediate EUh (— 3—6) and (iii) a high H-bond acceptor potential ( EUh > 10 i.e., transported slowly). 

The latter type of compounds should preferably carry either one type I unit or at most two units (positioned as far apart as possible), and have an elongated structure (which does not fold as verapamil, for example) with a small cross-sectional area, Ad- The first type of compounds is expected to be transported slowly, whereas the second type may not be transported. Table 20.2 summarizes the drug properties relevant for transporter binding and lipid partitioning of a substrate (modulator or inhibitor) of P-gp. Inspection of the information contained in Table 20.2 shows that the synthesis and membrane incorporation of inhibitors with a low number of H-bond acceptor patterns should be simpler and more efficient than that of inhibitors with a large number of patterns. 

P-gp is expressed in tumor tissue and serves as a barrier to increased accumulation of cytotoxic anticancer drugs within the cancer cell by actively pumping the drug out of the cell. It is a multidrug resistance element by virtue of the fact that it is promiscuous in terms of substrate selectivity. It, like the cytochromes P450, will accept a wide diversity of structural types. For example, a small sampling of drugs that are substrates for P-gp... 

Figure 16.8 Structures of three potent P-gp inhibitors taken from the literature [55].

Detailed three-dimensional models of P-gp-substrate complexes representing the various steps of the catalytic cycle would be significantly helpful in elucidating the molecular mechanism for substrate binding and release. The relatively poor 3D structural information available [14—16] and the complex mechanism for compounds undergoing P-gp-compound interactions explain why only a few groups have attempted to build and study 3D homology models for P-gp [56,58,60,70]. 

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