The present invention relates to small molecule modulators of G protein-coupled receptor six (GPR6); preferably, the small molecule modulators are preferentially selected for the human GPR6; most preferably, the small molecule modulators are inverse agonists to the human GPR6.
A. G Protein-Coupled Receptors
G protein-coupled receptors (GPCR) share a common structural motif. All these receptors have seven sequences of between 22 to 24 hydrophobic amino acids that form seven alpha helices, each of which spans the membrane. The transmembrane helices are joined by strands of amino acids having a larger loop between the fourth and fifth transmembrane helix on the extracellular side of the membrane. Another larger loop, composed primarily of hydrophilic amino acids, joins transmembrane helices five and six on the intracellular side of the membrane. The carboxy terminus of the receptor lies intracellularly with the amino terminus in the extracellular space. It is thought that the loop joining helices five and six, as well as the carboxy terminus, interact with the G protein. Currently, Gq, Gs, Gi, and Go are G proteins that have been identified.
Under physiological conditions, GPCRs exist in the cell membrane in equilibrium between two different states or conformations: an xe2x80x9cinactivexe2x80x9d state and an xe2x80x9cactivexe2x80x9d state. A receptor in an inactive state is unable to link to the intracellular transduction pathway to produce a biological response. Changing the receptor conformation to the active state allows linkage to the transduction pathway and produces a biological response.
A receptor may be stabilized in an active state by an endogenous ligand or an exogenous agonist ligand. Recent discoveries, including but not exclusively limited to, modifications to the amino acid sequence of the receptor, provide alternative mechanisms other than ligands to stabilize the active state conformation. These approaches effectively stabilize the receptor in an active state by simulating the effect of a ligand binding to the receptor. Stabilization by such ligand-independent approaches is termed xe2x80x9cconstitutive receptor activation.xe2x80x9d A receptor for which the endogenous ligand is unknown or not identified is referred to as an xe2x80x9corphan receptor.xe2x80x9d
B. Traditional Compound Screening
Generally, the use of an orphan receptor for screening purposes to identify compounds that modulate a biological response associated with such receptor has not been possible. This is because the traditional xe2x80x9cdogmaxe2x80x9d regarding screening of compounds mandates that the ligand for the receptor be known, whereby compounds that competitively bind with the receptor, i.e., by interfering or blocking the binding of the natural ligand with the receptor, are selected. By definition, then, this approach has no applicability with respect to orphan receptors. Thus, by adhering to this dogmatic approach to the discovery of therapeutics, the art, in essence, has taught and has been taught to forsake the use of orphan receptors unless and until the natural ligand for the receptor is discovered. The pursuit of an endogenous ligand for an orphan receptor can take several years and cost millions of dollars.
Furthermore, and given that there are an estimated 2,000 GPCRs in the human genome, the majority of which being orphan receptors, the traditional dogma castigates a creative approach to the discovery of therapeutics to these receptors.
Numerous orphan G protein-coupled receptors are constitutively active in their endogenous state. GPR6 is a 362 amino acid homolog of GPR3; the endogenous ligand for GPR6 is unknown (Song, Z.-H. et al, supra.; see FIG. 1 for reported amino acid sequence). GPR6 transcripts are reported to be abundant in the human putamen and to a lesser extent in the frontal cortex, hippocampus, and hypothalamus (Heiber, M. et al. DNA and Cell Biology (1995) 14(1): 25; see FIG. 1 for reported nucleic acid and amino acid sequences for GPR6).
C. Obesity
Recently, our current knowledge of human obesity has advanced dramatically. Previously, obesity was viewed as an oppugnant behavior of inappropriate eating in the setting of appealing foods. Studies of animal models of obesity, biochemical alterations in both humans and animals, and the complex interactions of psychosocial and cultural factors that create receptiveness to human obesity indicate that this disease in humans is multifaceted and deeply entrenched in biologic systems. Thus, it is almost certain that obesity has multiple causes and that there are different types of obesity.
An increasing number of children and adolescents are overweight. Although not all overweight children will necessarily become overweight adults, the growing occurrence of obesity in childhood is likely to be reflected in increasing obesity in adult years. The high prevalence of obesity in our adult population and the likelihood that the nation of the future will be even more obese demands a re-examination of the health implications of this disease. See, Health Implications of Obesity. NIH Consens. Statement Online 1985 February 11-13; 5(9):1-7.
xe2x80x9cClinical obesityxe2x80x9d is a measurement of the excess body fat relative to lean body mass and is defined as a body weight more than 20% above the ideal body weight. Recent estimates suggest that 1 in 2 adults in the United States is clinically obese, an increase of more than 25% over the past decades. Flegal M D., et al., 22 Int. J. Obes. Relat. Metab. Disor. 39 (1998). Both overweight conditions and clinical obesity are a major health concerns worldwide, in particular because clinical obesity is often accompanied by numerous complications, i.e., hypertension and Type II diabetes, which in turn can cause coronary artery disease, stroke, late-stage complications of diabetes and premature death. (See, e.g., Nishina P. M. et al., 43 Metab. 554 (1994)).
Although the etiologic mechanisms underlying obesity require further clarification, the net effect of such mechanisms leads to an imbalance between energy intake and expenditure. Both genetic and environmental factors are likely to be involved in the pathogenesis of obesity. These include excess caloric intake, decreased physical activity, and metabolic and endocrine abnormalities.
Treatment of overweight conditions and clinical obesity via pharmaceutical agents are not only of importance with respect to the conditions themselves, but also with respect to the possibility of preventing other diseases that are associated with, e.g., clinical obesity, as well as enhancement of the positive feeling of xe2x80x9cselfxe2x80x9d that often accompanies those who are overweight or clinically obese and who encounter a significant reduction in body weight.
Given the foregoing discussion, it is apparent that compounds which help in the treatment of such disorders would be useful and would provide an advance in both research and clinical medicine. The present invention is directed to these, as well as other, important ends.
The present invention relates to small molecule modulators of the GPR6 receptor. Most preferably, the GPR6 modulators have inverse agonist characteristics at the receptor.