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Trophogen, Inc., Rockville, Maryland
Received May 17, 2007; accepted May 23, 2007
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Abstract |
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 Top Abstract Evolutionary Adaptation of...Receptor Binding and EfficacyOther Factors Affecting EfficacyConcluding RemarksReferences |
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) provides an important advance in the GnRH antagonists field and an illustration of the fact that preclinical studies using animal models with nonhuman receptors may have very limited value in predicting drug efficacy in human disease. There are many examples showing that high-affinity protein, peptide, or nonpeptide agonists or antagonists have also enhanced clinical efficacy. However, there are also numerous studies indicating that very high receptor binding affinity is not a guarantee of drug efficacy and that other factors, including pharmacokinetic profile, ligand-induced receptor desensitization, and "trafficking," are critical in design and development of optimal drugs.
; Millar, 2005). GnRH I is more potent in the activation of the Gq/11 protein in the gonadotrope; however, GnRH II is more potent in the stimulation of apoptosis and antiproliferative effects through activating Gi protein-mediated signaling. GnRH binding also causes up-regulation and clustering of GnRH receptors, resulting in their internalization, partial degradation, and recycling. Activation of many GPCR receptors, including GnRH receptor, is likely to be associated with breaking, by a ligand, the intrareceptor interactions that stabilize the receptor in its inactive conformations, creating new inter- and intramolecular contacts, resulting in ligand-specific set of receptor conformations (Zhang et al., 2000; Lu et al., 2005).
During the 4 decades since initial identification and sequencing of GnRH by Schally et al. (1971), several thousand peptide and nonpeptide GnRH receptor agonists and antagonists have been synthesized and tested. Long-acting GnRH agonists and antagonists are currently used in assisted reproduction and a wide spectrum of sex-hormone-dependent diseases. GnRH agonists bind to pituitary GnRH receptors and initially induce release of FSH and LH ("flare"), which is followed by enhanced internalization of GnRH receptors and subsequent decrease of FSH and LH levels. Clinical efficacy of antagonists is based on their ability to bind GnRH receptor and prevent binding of endogenous GnRH by prolonged receptor occupancy (Broqua et al., 2002). In contrast to agonists, they act without induction of typical for GnRH receptor second messenger production and cellular responses.
Peptide and nonpeptide GnRH antagonists were developed in the last decade to overcome several limitations of the agonists, including "flare-up" phenomenon and limited efficacy of agonists. GnRH peptide analogs are currently widely used to treat very diverse clinical conditions, but very common application is still ovarian stimulation for in vitro fertilization (Macklon et al., 2006; Tarlatzis et al., 2006). However, many such antagonists have very poor oral activity, quite strong histamine-releasing properties, and a very high metabolic clearance rate, requiring multiple injections or depot formulation.
The first potent and orally active nonpeptide antagonist was reported in 1998 (Cho et al., 1998). Several improved GnRH antagonists, including TAK-013 (sufugolix), were later synthesized and partially characterized by researchers at the Takeda Chemical Industries (Osaka, Japan). TAK-013 entered clinical trials for endometriosis and uterine fibroids in the United States, Europe, and Japan. Researchers from Neurocrine Biosciences developed several new nonpeptide GnRH antagonists, including NBI-42902 (Tucci et al., 2005). Nonpeptide GnRH antagonists offer convenient oral administration, which is particularly important in long-term treatment of GnRH-dependent precocious puberty, endometriosis, and prostate cancer. Many new compounds are not only more potent and longer acting than older antagonists but also lack undesirable side effects related to histamine release.
In theory, action of orthosteric receptor antagonist on biological response can result in three different changes of agonist dose-response curves: (1) shifting to the right with no decrease of the maximal response (surmountable antagonism); (2) decrease of maximal response combined with a right shift in potency (insurmountable antagonism); and (3) decrease of maximal response without right shift in potency (insurmountable antagonism). Examples of the first two instances are presented in the article by Kohout et al. (2007). The authors determined amino acid residues involved in TAK-013 insurmountability at the human GnRH receptor that are absent in the Rhesus macaque GnRH receptor molecule. Comparison of GnRH receptor sequences and receptor mutagenesis combined with molecular modeling led the authors to the "trap door" hypothesis, explaining different behavior of TAK-013 at the human and Rhesus macaque receptors.
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Evolutionary Adaptation of Primate GnRH Receptors |
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TopAbstractEvolutionary Adaptation of...Receptor Binding and EfficacyOther Factors Affecting EfficacyConcluding RemarksReferences |
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-subunit was correlated with their decreased bioactivity (Szkudlinski et al., 1996, 2002). In the case of TSH, the attenuated bioactivity after separation of apes from Old World monkeys may be due to the greater need to conserve iodine for thyroid hormone synthesis during long periods of fasting in the nomadic life that early hominoids had to endure. Similar modifications of FSH, chorionic gonadotropin, and LH intrinsic activities in apes and humans compared with New World and Old World monkeys are probably related to slower rates of reproduction and maturation in hominoids (i.e., all apes and humans). It is noteworthy that the evolutionary rate of gonadotropin receptors is considerably slower than that of their natural ligands. In contrast to gonadotropin receptors, significant functional differences between human and Old World monkey GnRH receptors, as described by Kohout et al. (2007), may suggest adaptation of GnRH receptor to simultaneous modifications in other components of reproductive system during primate evolution. Consistent with more rigorous control of reproduction in apes and humans, significantly lower expression of human than rodent GnRH receptors was recently correlated with the presence of primate-specific lysine 191 residue, increased rate of receptor misfolding, and decreased receptor signaling (Janovick et al., 2006). Such restriction of human GnRH receptor cell membrane expression developed under strong selective pressure and quite complex pattern of cyclicity associated with primate reproduction (Janovick et al., 2007). Consequently, evolutionary changes resulted in a human GnRH receptor molecule that is precisely balanced between either expression at the plasma membrane or internalization and degradation in the endoplasmic reticulum. Such balance was proposed to be the reason why the human GnRH receptor, in contrast to its more highly expressed rodent counterparts, is highly susceptible to single-point mutations that result in disease (Conn et al., 2006). The challenge that lies before us is to get a deeper insight into many uniquely human or primate reproductive phenomena and integrate them with quite complex regulation of hormone and receptor function. |
Receptor Binding and Efficacy |
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TopAbstractEvolutionary Adaptation of...Receptor Binding and EfficacyOther Factors Affecting EfficacyConcluding RemarksReferences |
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; Szkudlinski et al., 2002, 2004; Schreiber et al., 2006). A widespread view is that the most crucial factor for sustained drug efficacy in vivo is not the apparent affinity for the target receptor, but rather the residency time at the receptor that is measured by the dissociation half-life (Copeland et al., 2006). It determines the longer drug effect in vitro or in vivo and is commonly considered a key parameter predicting duration of drug efficacy. Accordingly, Kohout et al. (2007) showed a direct correlation between the degree of insurmountability and the dissociation rate of TAK-013 from the receptor. It is possible that the trapping mechanism may function not only for nonpeptide ligands but also may be responsible for delayed dissociation of many peptide and protein ligands. Such a ligand-trapping mechanism could be enhanced in certain receptor systems with receptor dimerization, coreceptor recruitment, glycosaminoglycans, and other components of cell membrane. The question remains whether a higher degree of insurmountability results in enhanced clinical efficacy of TAK-013. In contrast to experiments performed in closed systems (e.g., static short-term in vitro studies), the clinical efficacy of a drug is influenced by many factors other than molecular determinants of ligand-receptor complex.
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Other Factors Affecting Efficacy |
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TopAbstractEvolutionary Adaptation of...Receptor Binding and EfficacyOther Factors Affecting EfficacyConcluding RemarksReferences |
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), it is expected that the efficacy of GnRH antagonist is also dependent on the rate of internalization affecting total number of GnRH receptors in the cell membrane. It was shown previously that certain GnRH agonists and antagonists are internalized to a similar extent and that receptor activation may not always be necessary to initiate the rapid internalization of hormone-receptor complexes (Hazum et al., 1983; Loumaye et al., 1984). In addition, it is also critical if the antagonist-receptor complex is routed after internalization into a recycling pathway or to a degradation pathway.
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Ligand-induced GPCR receptor dimerization and oligomerization has emerged recently as a factor modulating cellular signaling. Some studies indicate that GnRH agonist, but not antagonist, occupancy of the GnRH receptor promotes microaggregation of receptors (Cornea et al., 2001; Cornea and Conn, 2002). Cell specific colocalization of signaling components within lipid rafts and caveolae emerged as an additional modifier of drug responses (Ostrom and Insel, 2004). Cell surface receptor expression and receptor coupling to various effectors in the target organ is obviously critical for drug efficacy. Agonist potency and efficacy is known to decrease in systems with low receptor levels and impaired/altered effector coupling (Kenakin, 2003). If natural ligand X requires 500 receptors in a given cell to produce maximal response, whereas the high-affinity analog X requires 50, then a decrease of the receptor density to 50 per cell should theoretically not decrease maximal response to analog X, but only to the natural ligand.
In addition to the assessments based on concentration of the drug in the blood, drug concentration at the target cells depends on organ perfusion and drug distribution from blood to a given tissue. Receptor spliced variants, altered signaling mechanism, modulation of multiple targets, and unanticipated effects of ligand modification may create even more complexity in the process of predicting clinical efficacy (Sarkar et al., 2002). Based on research and development experiences with many drug candidates, it is now becoming abundantly clear that the clinical efficacy and safety in humans is never completely certain, even after completion of clinical trials.
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Concluding Remarks |
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TopAbstractEvolutionary Adaptation of...Receptor Binding and EfficacyOther Factors Affecting EfficacyConcluding RemarksReferences |
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) strongly suggest that using inherent receptor trapping properties may lead to design of novel insurmountable antagonists within GPCRs and other receptor superfamilies. Perhaps an era of highly specific and efficacious insurmountable receptor antagonists is just beginning. |
Footnotes |
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ABBREVIATIONS: GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone; FSH, follicle-stimulating hormone; NBI-42902, 3-[(2R)-amino-2-phenylethyl]-1-(2,6-difluorobenzyl)-5-(2-fluoro-3-methoxyphenyl)-6-methylpyrimidin-2,4-dione; TSH, thyroid-stimulating hormone.
Address correspondence to: Mariusz W. Szkudlinski, Trophogen, Inc., 6 Taft Court, Suite 150, Rockville, Maryland 20850. E-mail: mszkudlinski{at}trophogen.com
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References |
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