A. G Protein-Coupled Receptors
Although a number of receptor classes exist in humans, by far the most abundant and therapeutically relevant is represented by the G protein-coupled receptor (GPCR) class. It is estimated that there are some 30,000-40,000 genes within the human genome, and of these, approximately 2% are estimated to code for GPCRs. Receptors, including GPCRs, for which the endogenous ligand has been identified are referred to as “known” receptors, while receptors for which the endogenous ligand has not been identified are referred to as “orphan” receptors.
GPCRs represent an important area for the development of pharmaceutical products: from approximately 20 of the 100 known GPCRs, approximately 60% of all prescription pharmaceuticals have been developed. For example, in 1999, of the top 100 brand name prescription drugs, the following drugs interact with GPCRs (the primary diseases and/or disorders treated related to the drug is indicated in parentheses):
Claritin ® (allergies)Prozac ® (depression)Vasotec ® (hypertension)Paxil ® (depression)Zoloft ® (depression)Zyprexa ® (psychotic disorder)Cozaar ® (hypertension)Imitrex ® (migraine)Zantac ® (reflux)Propulsid ® (reflux disease)Risperdal ® (schizophrenia)Serevent ® (asthma)Pepcid ® (reflux)Gaster ® (ulcers)Atrovent ® (bronchospasm)Effexor ® (depression)Depakote ® (epilepsy)Cardura ® (prostatic ypertrophy)Allegra ® (allergies)Lupron ® (prostate cancer)Zoladex ® (prostate cancer)Diprivan ® (anesthesia)BuSpar ® (anxiety)Ventolin ® (bronchospasm)Hytrin ® (hypertension)Wellbutrin ® (depression)Zyrtec ® (rhinitis)Plavix ® (MI/stroke)Toprol-XL ® (hypertension)Tenormin ® (angina)Xalatan ® (glaucoma)Singulair ® (asthma)Diovan ® (hypertension)Harnal ® (prostatic hyperplasia)(Med Ad News 1999 Data).
Generally, when an endogenous ligand binds with the receptor (often referred to as “activation” of the receptor), there is a change in the conformation of the intracellular region that allows for coupling between the intracellular region and an intracellular “G-protein.” It has been reported that GPCRs are “promiscuous” with respect to G proteins, i.e., that a GPCR can interact with more than one G protein. See, Kenakin, T., 43 Life Sciences 1095 (1988). Although other G proteins exist, currently, Gq, Gs, Gi, Gz and Go are G proteins that have been identified. Ligand-activated GPCR coupling with the G-protein initiates a signaling cascade process (referred to as “signal transduction”). Under normal conditions, signal transduction ultimately results in cellular activation or cellular inhibition. Although not wishing to be bound to any theory, it is thought that the IC-3 loop as well as the carboxy terminus of the receptor interact with the G protein.
Under physiological conditions, GPCRs exist in the cell membrane in equilibrium between two different conformations: an “inactive” state and an “active” state. A receptor in an inactive state is unable to link to the intracellular signaling transduction pathway to initiate signal transduction leading to a biological response. Changing the receptor conformation to the active state allows linkage to the transduction pathway (via the G-protein) and produces a biological response.
A receptor may be stabilized in an active state by a ligand or a compound such as a drug. Recent discoveries, including but not exclusively limited to modifications to the amino acid sequence of the receptor, provide means other than ligands or drugs to promote and stabilize the receptor in the active state conformation. These means effectively stabilize the receptor in an active state by simulating the effect of a ligand binding to the receptor. Stabilization by such ligand-independent means is termed “constitutive receptor activation.”
B. CXC Chemokine Receptor Subtype-3 (“CXCR3”)
The CXC chemokine receptor subtype 3 (CXCR3) was cloned and originally referred to as GPR9 in 1995 and mapped incorrectly to human chromosome 8p11.2-12 (Marchese et al., 23, 609-618 (1995)) but was later mapped correctly to Xq13 (Loetscher et al., Eur J Immunol 28: 3696-3705 (1998)). After much analysis of GPR9, this receptor was identified to bind to one or more members of the chemokine superfamily of chemotatic cytokines and received its new nomenclature as CXCR3. CXCR3 was identified to bind to three agonists, γ-interferon inducible protein-10 (“IP-10”), interferon inducible T cell α-chemoattractant (“I-TAC”), and monoline induced by γ-inteferon (“Mig”). Murphy, P. M., et al., Pharmacol Rev 2000 March;52(1): 145-76.
CXCR3 has been determined to be expressed in circulating blood T-cells, B-cells, natural killer (NK) cells, and eosinophils. An increase in the expression levels and responsiveness of the CXCR3 is detected when T cells are active. This observation revealed that CXCR3 is involved in inflammation, as well as, leukocyte trafficking and immune surveillance.