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

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 assay 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.

High Affinity Antagonists of the Vanilloid Receptor

“…Piperine has been reported to be pungent but not to desensitize (Liu and Simon, 1996). Several capsaicin analogs with a phorbol ester as a C region were inactive for modulating thermoregulation, although they were potent for neurogenic inflammation Appendino et al, 1996). The loss of the range of vanilloid responses in mice null for VR1 argues against the otherwise attractive model that multiple genes for the vanilloid receptor exist (Caterina et al, 2000;Davis et al, 2000).…”

Synthesis and Biological Evaluation of Phorbol‐Resiniferatoxin (RTX) Hybrids

“…[12] The new location of the bridge forces ring C into a boat-like conformation, and the closure of a semiacetal ring with the 9-hydroxy group of 6a further contributes to the overall mimicry of the conformation of RTX. A precursor for the α-ketol rearrangement (4b) was prepared from 13-acetylphorbol 20-trityl ether (3c) [9] by PCC oxidation of the secondary C-12 hydroxy group. Surprisingly, the rearrangement of 4b took different courses in methanol and in ethanol.…”

“…The regiochemistry of esterification was verified by NOE experiments carried out on the final 20-homovanillates 5d and 6d, obtained by a previously established procedure (removal of the trityl group with HClO 4 , esterification with Mem-protected homovanillic acid, and SnCl 4 -mediated deprotection of the phenolic hydroxy group). [9] The NOE experiments showed that in both 5d and 6d the phenylacetyl group was bound to the hydroxy group at C-12 [NOEs between the phenyl protons and the α-oriented methyl groups (C-18 and C-16)]. Alternative locations for the phenylacetyl residue, namely at the C-9 hydroxy group for the phorbobutanone homovanillate (5d) or at the semiacetal oxygen for the phorboisobutyl ketone homovanillate (6d), would have involved lack of correlation with the methyl group (C-16) (esterification of the 10-hydroxy group of 5d) or with the methyl group (C-18) (esterification of the semiacetalic hydroxy group of 6d).…”

“…Our first efforts in this area resulted in the discovery of a series of phorbol-RTX hybrids endowed with vanilloid activity. [9] These compounds, exemplified by PPAHV (3b), raised interest because of their unique behaviour in binding and functional assays of vanilloid activity. [9,10] Since the vanilloid profile of phorboid homovanillates is distinct from those of both capsaicinoids and resiniferonoids, [10] it was interesting to expand their structure-activity relationships.…”

“…[9] These compounds, exemplified by PPAHV (3b), raised interest because of their unique behaviour in binding and functional assays of vanilloid activity. [9,10] Since the vanilloid profile of phorboid homovanillates is distinct from those of both capsaicinoids and resiniferonoids, [10] it was interesting to expand their structure-activity relationships. Thus, taking PPAHV as a lead, we have focused on modifications of ring C to mimic the constitution and conformation of resiniferatoxin, and on changes of the AB ring system to provide phorboid platforms devoid of tumour-promoting potential.…”