Synthesis and Evaluation of Phorboid 20-Homovanillates: Discovery of a Class of Ligands Binding to the Vanilloid (Capsaicin) Receptor with Different Degrees of Cooperativity

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Abstract
A number of phorboid 20-homovanillates were prepared by condensation of phorbol 12,13-diesters and 12-dehydrophorbol 13-esters with Mem-homovanillic acid followed by removal of the protecting group with SnCl4 in THF. These compounds were evaluated for their ability to inhibit [3H]resiniferatoxin (RTX) binding to rat spinal cord membranes. Compounds bearing a lipophilic ester group on ring C were considerably active, but a surprising tolerance of the vanilloid receptor toward the location and the orientation of this ester group was disclosed. Unexpectedly, these ligands could also diminish, to a variable degree, the positive cooperativity which characterizes RTX binding to the vanilloid receptor. Phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV, 6a), a compound which abolished binding cooperativity, was further tested in a variety of in vivo assays used to characterize vanilloid-like activity. PPAHV showed only a marginal pungency and failed to induce a measurable hypothermia response at doses (up to 200 mg/kg) at which it effectively desensitized against neurogenic inflammation. These data suggest that the peculiar binding behavior of these ligands might be associated with a distinct spectrum of biological activity.