Linda S. Williams

Further studies on ocular responses to DP receptor stimulation

Prostaglandin D2 (PGD2) and the selective DP receptor agonist BW 245C have been previously shown to lower intraocular pressure in rabbits, while PGD2, but not BW 245C, caused plasma extravasation, eosinophil infiltration, and goblet cell depletion. In these present studies definition of the ocular pharmacology of prostaglandin D2 (PGD2) has been extended by using a further selective DP receptor agonist SQ 27986 and a potent and selective DP receptor antagonist BW A868C. In cats and rabbits SQ 27986 caused ocular hypotension. The ocular hypotensive effect of PGD2 in rabbits was blocked by pretreatment with the DP receptor antagonist BW A868C, whereas the activities of PGE2 and PGF2 alpha remained unaltered. The singular involvement of the DP receptor in changes in rabbit intraocular pressure evoked by PGD2 was thereby verified by using the antagonist BW A868C. In terms of effects on the ocular surface, SQ 27986 caused no increase in conjunctival microvascular permeability, no eosinophil infiltration, and no depletion of the goblet cell population. These findings reinforce the concept that selective DP receptor agonists may be useful for lowering intraocular pressure without causing ocular surface pathology. PGD2 induced increases in conjunctival microvascular permeability were inhibited by BW A868C, despite the fact that DP receptor agonists failed to evoke a plasma exudation response. This finding was unexpected and suggests a possible subdivision of the DP receptor designation.

Evidence for human thromboxane receptor heterogeneity using a novel series of 9,11-cyclic carbonate derivatives of prostaglandin F2α

1 The pharmacological activity of a novel series of 9,11‐cyclic carbonate derivatives of prostaglandin F2α (PGF2α) was investigated in various isolated smooth muscle preparations possessing different prostanoid receptor subtypes as well as in human platelets. Since subdivision of thromboxane (TP‐) receptors into vascular/smooth muscle and platelet subtypes is a controversial subject, our studies included a human smooth muscle preparation (myometrium) in addition to the widely used rat aorta and human platelets as TP‐receptor preparations. 2 Two members of that series, AGN191976 and AGN192093 were found to be highly potent and selective thromboxane‐mimetics. AGN191976 and AGN192093 contracted isolated tissues of the rat thoracic aorta with EC50 values of 0.32±0.08 and 1.30±0.53 nM, respectively. Both agonists were at least 10 times more potent than the benchmark TP‐agonist, U‐46619, in this preparation, whilst being at least 500 times less potent at other prostanoid receptors (DP, EP1, EP3, FP, IP) in vitro. 3 In human myometrial strips from pregnant and non‐pregnant donors, both AGN191976 and AGN192093 were potent contractile agonists. The rank order of potency in myometrium of AGN191976>AGN192093>U‐46619 correlated well with that in the rat aorta. In human plateletrich plasma (PRP), however, AGN191976 had potent proaggregatory activity (EC50=16.3±1.4 nM), which is a TP‐receptor‐mediated event, whereas AGN192093 was a much weaker agonist (EC50= 37.9±2.0 μm). AGN192093 did not behave as an antagonist in the platelets, since it did not antagonize platelet aggregation induced by ADP, arachidonic acid, U‐46619 or AGN191976. In human washed platelets, the activity profile of AGN191976 (EC50=4.15±0.52 nM) and AGN192093 (no aggregation up to 10 μm) was similar to that obtained in PRP. 4 The involvement of TP‐receptors was verified with the potent TP‐antagonist, SQ29548. SQ29548 (0.1 μm in myometrium; 1 μm in aorta; 1 μm and 10 μm in platelets) antagonized responses to U‐46619, AGN191976 and AGN192093 as expected. 5 In conclusion, AGN191976 and AGN192093, both 9,11‐cyclic carbonate derivatives of PGF2α, were found to be highly potent and selective thromboxane‐mimetics in rat vascular and human myometrial smooth muscle. However, only AGN 191976 was a potent agonist at TP‐receptors in human platelets. The differential activity of AGN192093 on TP‐receptor‐mediated events in platelets and smooth muscle provides further evidence for a subdivision of TP‐receptors. AGN192093 appears to be a useful tool for the pharmacological distinction of TP‐receptor subtypes.

Identification of 19 (R)-OH prostaglandin E2 as a selective prostanoid EP2-receptor agonist

The physiological significance of the formation of large quantities of 19(R)-hydroxy prostaglandin Es (19-OH PGE) from PGE1 and PGE2 in human seminal plasma is intriguing. The concept that prostaglandins exert their biological effects by interacting with specific receptors, according to the current working classification for prostanoid receptors, was employed as a conceptual framework to re-examine the activity of 19(R)-OH PGs. In contrast to PGE2, which may indiscriminately stimulate a variety of prostanoid receptor subtypes, 19(R)-OH PGE2 exhibited selectivity for the EP2-receptor subtype. In EP1 (guinea pig ileum contraction), EP2 (cat trachea relaxation), and EP3 (chick ileum contraction) preparations where PGE2 is equipotent, 19(R)-OH PGE2 exhibited greater potency in the EP2-receptor population. Moreover, unlike PGE2, 19(R)-OH PGE2 did not stimulate an FP-receptor preparation (cat iris). 19(R)-OH PGE2 was devoid of activity at thromboxane A2-(TP), prostaglandin D2-(DP) and prostacyclin-(IP) sensitive receptors as indicated by its inability to cause human platelet aggregation or inhibit ADP-induced platelet aggregation. 19(R)-OH PGE1 had an entirely converse profile of activity. As a myotropic agent in the guinea pig and chick ileal preparations, 19(R)-OH PGE1 was approximately 1.5 orders of magnitude more potent than 19(R)-OH PGE2 but it appeared devoid of EP2-receptor stimulant properties. 19(R)-OH PGF2 alpha possessed very little biological activity in a diverse variety of isolated tissue preparations, indicating that 19-hydroxylation represents a highly efficient inactivation step for PGF2 alpha. The implications of the formation of receptor selective PGE derivatives in human seminal fluid for human reproductive physiology remains to be established.

Interactive effects of peptidoleukotrienes and histamine on microvascular permeability and their involvement in experimental cutaneous and conjunctival immediate hypersensitivity

The involvement of peptidoleukotrienes (LTs) in mediating the increase in microvascular permeability associated with experimental cutaneous immediate hypersensitivity was studied by examining the effect of SK&F 104353, a potent and selective LT-antagonist, on the response evoked by graded, intradermal injections of antigen. SK&F 104353, employed at doses that profoundly blocked LTC4, LTD4 and LTE4 responses, significantly reduced the response produced by experimental cutaneous immediate hypersensitivity. The response to the lowest antigen dose (0.1 microgram) was, however, entirely insusceptible to SK&F 104353. The effect of SK&F 104353 was also examined in combination with a pyrilamine-cimetidine dosing regimen sufficient to remove the histaminergic component of cutaneous immediate hypersensitivity. The non-histaminergic component associated with higher antigen doses (10 and 100 micrograms) was significantly reduced but not abolished by SK&F 104353; the non-histaminergic component associated with low antigen doses (0.1 and 1 microgram) was not susceptible to SK&F 104353. Thus, the increase in cutaneous microvascular permeability evoked by immediate hypersensitivity appears to comprise three components: (1) A histaminergic response apparent for all antigen doses; (2) a LT-mediated component which is manifest in response to high antigen doses; (3) a third, unidentified component that is present for the entire antigen dose-range but contributes less to the overall response when high antigen doses are used. A distinct non-histaminergic, non-leukotriene mediated component was not a feature of conjunctival immediate hypersensitivity. SK&F 104353 administered in combinatio with pyrilamine-cimetidine virtually abolished the response with a small residual remaining only for the highest antigen dose. In further contrast to cutaneous immediate hypersensitivity, SK&F 104353 alone was comparatively ineffective in type 1 allergic conjunctivitis. This difference in susceptibility to SK&F 104353 appears to reflect the type of histamine-LTD4 interactive effect on microvascular permeability. Histamine and LTD4 were additive in terms of cutaneous microvascular permeability. In the conjunctiva, histamine and LTD4 appeared mutually exclusive in that the level of response produced by the combination tended not to exceed that of the single component which caused the greater effect.

Platelet-activating factor causes goblet cell depletion in the conjunctiva

Platelet-activating factor (PAF) (1-O-hexadecyl-2-acetyl-sn-glyceryl-3-phosphorylcholine) produced dose-dependent depletion of the goblet cell population associated with the conjunctival epithelium. Reductions in goblet cell numbers did not correspond to leukocyte infiltration and were consistent with a direct effect of PAF. In contrast, LTD4 and LTE4 did not affect the goblet cell population although they caused massive eosinophil infiltration into the conjunctival epithelium. Histamine also produced conjunctival goblet cell depletion, but this appeared secondary to eosinophil degranulation and resultant epithelial desquamation. In addition to goblet cell expulsion, PAF produced an increase in conjunctival microvascular permeability over an identical dose-range. PAF-induced leukocyte emigration was small or absent and comprised a neutrophil infiltrate which exhibited no clear dose-dependent relationship. Lyso-PAF produced effects only at the highest dose employed where pathological changes and a distinct increase in conjunctival microvascular permeability were evident. Lyso-PAF- and PAF-induced increases in conjunctival microvascular permeability were virtually abolished by the PAF antagonist CV-6209. The pronounced inhibitory activity of CV-6209 suggests that high doses of lyso-PAF may either weakly stimulate conjunctival PAF receptors or that there may be sufficient conversion of lyso-PAF to biologically active levels of PAF.

Ocular Hypotensive Activity of Prostaglandin F2α (PGF2α) Analogs with Neutral Substituents at Position 1 is Predicted by the Isolated Cat Iris Sphincter Smooth Muscle Preparation but not Ca+ Signalling in Swiss 3T3 Cells

Typically, the C-1 carboxylic acid (CO2H) has been considered an essential pharmacophoric group for ligand binding to the FP-receptor. Historically, in drug design and development, a formal negatively charged carboxylic acid can be replaced with a group of similar pKa, such as tetrazole or an acyl sulfonamide. This change sometimes provides better selectivity and/or increased potency due to better metabolic stability. We investigated replacement of the C-1 carboxylic acid with a variety of uncharged C-1 substituents. A diverse series of Cl-analogs was synthesized and certain compounds, especially AGN-191129 (PGF2aOCH3), exhibited an apparent unique pharmacology, thus confiming the importance of the Cl-CO2H for binding to the FP-receptor.