Synthesis and pharmacological activity of B3-adrenoceptor ligands

The B3-adrenoceptor (B-AR) was first classified in 1984 in rat brown adipose tissue. The classification of this receptor in other tissues and species is hampered by the lack of selective B3-antagonists which, despite the identification of further classes of B3-agonist ligands, remains elusive. In this study, a series of novel B3-AR antagonist drugs were synthesised and their pharmacological profile in rat ileum investigated with the aim of increasing our understanding of the structural requirements of drug-receptor binding in B3-ARs. Analogues of iodocyanopindolol (ICYP) (17) and conformationally impaired analogues of BRL 37344 were identified as key synthetic targets.

ICYP and eleven analogues were synthesised from epoxide precursor (38). The pharmacological activity of these compounds was determined in a rat ileum preparation with tissue contraction solely due to the B3-AR. All ICYP analogues were active at the B3-AR. The pharmacological data revealed: (i) of the analogues tested, ICYP (17) and CYP (18) had the highest affinities at the B3-AR. This is in sharp contrast to the binding of ICYP (17) and CYP (18) at B1- and B2-ARs where ICYP (17) has a hundred fold higher affinity than CYP (18); and (ii) two pharmacological characteristics determined for the drugs, namely pD2 and the pKb, were significantly different for eight of the analogues studied. These drugs were partial agonists, and the discrepancy between pD2 and pKb values indicated binding to more than one receptor population.

Three hypotheses were proposed to explain this observation; (i) two different B3-ARs are present; (ii) the result is an enantiomeric effect; and (iii) the difference is a non-specific lipophilic effect.

Log P values for the series were determined using HPLC, and no correlation was found between Log P and pD2 or pKb values.

To examine the second hypothesis, both enantiomers of CYP (18) and bupranolol (8) were synthesised and their pharmacological activity investigated in rat ileum. All four enantiomers tested were antagonists at the B3-AR, with the receptor displaying stereoselectivity for the (S)-enantiomers, with (S)-CYP (18b) being the most potent B3-antagonist drug identified. Examination of the partial agonist activity of CYP and bupranolol enantiomers suggested that this effect was independent of the mechanism of B3-antagonism. The partial agonist effect was selective for (R)-CYP (18a) and non-selective for bupranolol. This result clearly defines (S)-CYP (18b) as a potent B3-AR antagonist and highlights structure-activity studies of CYP analogues as an important new source of information for the design of new classes of B3-antagonist drugs.

To further investigate the structure-function relationship being developed for B3-AR selective drugs the "extended conformation" hypothesis of Blin and co-workers was examined. These researchers proposed that the discrepancies observed between the B3-AR activity, and the B1/B2-AR activity of known agonist and antagonist drugs resulted from the ability of B3-agonist drugs to adopt an extended conformation at the B3-AR. To investigate this postulate, conformationally impaired analogues of the B3-AR agonist BRL 37344 (6) were targeted for synthesis. Allylic amine (112) was identified as a key precursor for conformationally impaired BRL 37344 analogues and was synthesised with solely (E) geometry from (d)-Bocalinal (109) and 4-methoxy benzylphosphonium chloride (111). Progress in the synthesis of other subtargets is described.