Dear Editor,

We thank Dr. Camras for his interest in our report on levels of bimatoprost and its free acid in the aqueous humour of cataract patients after a single topical dose of bimatoprost [1] and welcome the opportunity to respond to his comments. We are in agreement with Dr. Camras that the results of our study [1] and those of his previously reported study [2] are similar, showing low nanomolar concentrations of 17-phenyl PGF2alpha (bimatoprost acid) in the aqueous humour. There is no question that bimatoprost acid is a metabolite of bimatoprost. The issue is whether bimatoprost acid levels account for the IOP-lowering activity of bimatoprost. The evidence suggests that they are insufficient to do so. As Dr. Camras stated in his correspondence: “bimatoprost yields peak free acid concentrations in the aqueous 3 to 6 times lower than latanoprost acid”. It is accepted that the biological effects of latanoprost are exerted by latanoprost acid: the relatively high concentration of latanoprost acid achieved after latanoprost dosing [1-3] is sufficient to activate a substantial proportion of the prostaglandin FP receptors present in the target tissues and account for the IOP-lowering effect of latanoprost. It is difficult to understand, however, how the much lower levels of bimatoprost acid achieved could be believed to account for the IOP-lowering effect of bimatoprost, particularly since bimatoprost has reduced IOP more effectively than latanoprost in some clinical studies [4,5] and is effective in patients who fail to respond to latanoprost [6,7].

Dr. Camras contends that there is overwhelming evidence that bimatoprost acid is more potent than latanoprost acid at prostaglandin FP receptors, but the 10 references he cites to support this statement [8-17] include 3 studies in which bimatoprost acid and latanoprost acid were not compared [8-10], 3 reviews from a single laboratory that reported greater functional potency of bimatoprost acid compared with latanoprost acid in the cat iris sphincter preparation [11-13], and a study that found 3-fold lower functional potency of bimatoprost acid compared with latanoprost acid in human trabecular meshwork cells (EC50 of 112 nM vs 34.7 nM) [14]. Dr. Camras fails to note that the study showing very high functional potency of bimatoprost acid in human HEK-293 cells [15] used transfected cells with overexpression of the FP receptor. Bimatoprost acid was reported to be very potent in stimulating phosphoinositide hydrolysis in nontransfected mouse 3T3 and rat A7r5 cells [16]. However, in other studies by the same laboratory, bimatoprost acid was 10-fold less potent in mobilizing intracellular Ca++ in these cell lines [9,18]. Reported potency values in human ciliary muscle cells were 3.8 nM and 3.6 nM for bimatoprost acid and 124 nM and 198 nM for latanoprost acid [16,17], but bimatoprost acid was less effective than latanoprost acid in stimulating MAP kinase at 100 nM [17]. In summary, review of the literature does not reveal compelling evidence that bimatoprost acid is more potent than latanoprost acid. In fact, in human trabecular meshwork cells as well as human fibroblasts expressing endogenous, nontransfected prostaglandin FP receptors, bimatoprost acid and latanoprost acid have shown similar functional potency [1,14,16]. The results with trabecular meshwork cells are more clinically relevant because one pathway by which bimatoprost is believed to reduce IOP is through effects on the trabecular meshwork [19]. Wan et al [20] have shown that bimatoprost produces a decrease in outflow facility, which is blocked by a prostamide antagonist, in a human anterior segment organ culture model.

Camras et al proposed that the 22 nM aqueous humour concentration of bimatoprost acid is sufficient to lower IOP based on its agonist potency. Following his line of reasoning, the 100 nM aqueous humour concentration reported for latanoprost acid should not be sufficient to account for the IOP lowering, based on EC50 potency values of 124 nM and 198 nM in ciliary muscle cells. In fact, the data suggest that aqueous humour concentrations of bimatoprost acid are not sufficient to activate the FP receptor for effective diurnal IOP lowering, particularly taking into account the aqueous humour concentration of latanoprost acid and respective agonist potencies. The most plausible explanation for the greater efficacy of bimatoprost, despite lower levels of bimatoprost acid in the aqueous humor, is that bimatoprost reduces IOP through a mechanism other than or in addition to production of bimatoprost acid. There is excellent evidence from animal studies that the intact bimatoprost molecule has biological activity distinct from the activity of prostaglandin FP agonists [21]. For example, in a dissociated cat iris preparation, a specific population of cells responds to bimatoprost with an increase in calcium levels, and a separate and distinct population of cells responds to bimatoprost acid with an increase in calcium levels [22]. The selective stimulation of different cells in the same preparation by bimatoprost and prostaglandin FP agonists suggests the involvement of receptors for bimatoprost distinct from prostaglandin FP receptors. The recent identification of an antagonist that blocks the effects of bimatoprost, but not bimatoprost acid or latanoprost acid, in the cat iris preparation has provided additional evidence for biological activity of bimatoprost mediated through novel receptors [23]. Although studies in prostaglandin FP receptor knockout mice have shown that the intact FP receptor gene is needed for the IOP response to bimatoprost in the mouse eye [24,25], the IOP response does not appear to be mediated by interaction of bimatoprost acid with prostaglandin FP receptors, because there is minimal hydrolysis of bimatoprost in the mouse eye [25]. Instead, intact bimatoprost may interact with an alternatively spliced prostaglandin FP receptor pharmacologically distinct from the well-characterized FP receptor [26].

The mechanism of action of bimatoprost is of considerable interest because bimatoprost appears to be the most effective medication now available for reducing IOP [4,27]. Further drug discovery may well aim to develop drugs that take advantage of a similar mechanism of action. For this reason it is important to consider the data from both clinical and laboratory studies and to be open-minded in reaching reasonable conclusions. To assume that the mechanism of action of bimatoprost is the same as that of latanoprost and ignore or misinterpret evidence inconsistent with that assumption is neither good science nor helpful to clinical advancements in lowering IOP.

References

1. Cantor LB, Hoop J, Wudunn D, et al. Levels of bimatoprost acid in the aqueous humour after bimatoprost treatment of patients with cataract. Br J Ophthalmol 2007;91:629-32.

2. Camras CB, Toris CB, Sjoquist B, et al. Detection of the free acid of bimatoprost in aqueous humor samples from human eyes treated with bimatoprost before cataract surgery. Ophthalmology 2004;111:2193-8.

3. Sjöquist B, Stjernschantz J. Ocular and systemic pharmacokinetics of latanoprost in humans. Surv Ophthalmol 2002;47(Suppl 1):S6-12.

4. Denis P, Lafuma A, Khoshnood B, et al. A meta-analysis of topical prostaglandin analogues intra-ocular pressure lowering in glaucoma therapy. Curr Med Res Opin 2007;23:601-8.

5. Dirks MS, Noecker RJ, Earl M, et al. A 3-month clinical trial comparing the IOP-lowering efficacy of bimatoprost and latanoprost in patients with normal-tension glaucoma. Adv Ther 2006;23:385-94.

6. Gandolfi SA, Cimino L. Effect of bimatoprost on patients with primary open-angle glaucoma or ocular hypertension who are nonresponders to latanoprost. Ophthalmology 2003;110:609-14.

7. Williams RD. Efficacy of bimatoprost in glaucoma and ocular hypertension unresponsive to latanoprost. Adv Ther 2002;19:275-81.

8. Sharif NA, Kelly CR, Williams GW. Bimatoprost (Lumigan®) is an agonist at the cloned human ocular FP prostaglandin receptor: real-time FLIPR-based intracellular Ca2+ mobilization studies. Prostaglandins Leukot Essent Fatty Acids 2003;68:27-33.

9. Sharif NA, Williams GW, Kelly CR. Bimatoprost and its free acid are prostaglandin FP receptor agonists. Eur J Pharmacol 2001;432:211-3.

10. Kelly CR, Williams GW, Sharif NA. Real-time intracellular Ca2+ mobilization by travoprost acid, bimatoprost, unoprostone, and other analogs via endogenous mouse, rat, and cloned human FP prostaglandin receptors. J Pharmacol Exp Ther 2003;304:238-45.

11. Stjernschantz JW. From PGF2a-isopropyl ester to latanoprost: a review of the development of Xalatan: the Proctor Lecture. Invest Ophthalmol Vis Sci 2001;42:1134-45.

12. Resul B, Stjernschantz J, Selén G, et al. Structure-activity relationships and receptor profiles of some ocular hypotensive prostanoids. Surv Ophthalmol 1997;41(Suppl 2):S47-S52.

13. Stjernschantz J, Albert D, Hu D, et al. Mechanism and clinical significance of prostaglandin-induced iris pigmentation. Surv Ophthalmol 2002;47(Suppl 1):S162-S175.

14. Sharif NA, Kelly CR, Crider JY. Human trabecular meshwork cell responses induced by bimatoprost, travoprost, unoprostone, and other FP prostaglandin receptor agonist analogues. Invest Ophthalmol Vis Sci 2003;44:715-21.

15. Sharif NA, Kelly CR, Crider JY. Agonist activity of bimatoprost, travoprost, latanoprost, unoprostone isopropyl ester and other prostaglandin analogs at the cloned human ciliary body FP prostaglandin receptor. J Ocul Pharmacol Ther 2002;18:313-24.

16. Sharif NA, Kelly CR, Crider JY, et al. Ocular hypotensive FP prostaglandin (PG) analogs: PG receptor subtype binding affinities and selectivities, and agonist potencies at FP and other PG receptors in cultured cells. J Ocul Pharmacol Ther 2003;19:501-15.

17. Sharif NA, Crider JY, Husain S, et al. Human ciliary muscle cell responses to FP-class prostaglandin analogs: phosphoinositide hydrolysis, intracellular Ca2+ mobilization and MAP kinase activation. J Ocul Pharmacol Ther 2003;19:437-55.

18. Kelly CR, Williams GW, Sharif NA. Real-time intracellular Ca2+ mobilization by travoprost acid, bimatoprost, unoprostone, and other analogs via endogenous mouse, rat, and cloned human FP prostaglandin receptors. J Pharmacol Exp Ther 2003;304:238-45.

19. Brubaker RF. Mechanism of action of bimatoprost (Lumigan). Surv Ophthalmol 2001;45(Suppl 4):S347-51.

20. Wan Z, Woodward DF, Stamer WD. Bimatoprost effects on conventional drainage tissues. Invest Ophthalmol Vis Sci 2007;48:E- Abstract 3919. Full manuscript in press.

21. Chen J, Senior J, Marshall K, et al. Studies using isolated uterine and other preparations show bimatoprost and prostanoid FP agonists have different activity profiles. Br J Pharmacol 2005;144:493-501.

22. Spada CS, Krauss AH, Woodward DF, et al. Bimatoprost and prostaglandin F(2 alpha) selectively stimulate intracellular calcium signaling in different cat iris sphincter cells. Exp Eye Res 2005;80:135- 45.

23. Woodward DF, Krauss AH, Wang JW, et al. Identification of an antagonist that selectively blocks the activity of prostamides (prostaglandin-ethanolamides) in the feline iris. Br J Pharmacol 2007;150:342-52.

24. Ota T, Aihara M, Narumiya S, et al. The effects of prostaglandin analogues on IOP in prostanoid FP-receptor-deficient mice. Invest Ophthalmol Vis Sci 2005;46:4159-63.

25. Crowston JG, Lindsey JD, Morris CA, et al. Effect of bimatoprost on intraocular pressure in prostaglandin FP receptor knockout mice. Invest Ophthalmol Vis Sci 2005;46:4571-7.

26. Liang Y, Li C, Guzman VM, Woodward DF. Identification of alternatively spliced variants of human prostaglandin FP receptor mRNA [Abstract 639.4]. Presented at: Experimental Biology 2005 and XXXV International Congress of Physiological Sciences, March 31-April 6, 2005; San Diego, CA.

27. Cantor LB, Hoop J, Morgan L, et al. Intraocular pressure-lowering efficacy of bimatoprost 0.03% and travoprost 0.004% in patients with glaucoma or ocular hypertension. Br J Ophthalmol 2006;90:1370-3.

Dear Editor,

We congratulate the authors of their adequately designed study(1) that demonstrates the high concentration of the free acid (the product of hydrolysis) of bimatoprost (BP), an amide, in the aqueous humor of patients receiving a single drop of BP 1, 3, or 6 hours prior to cataract surgery. This important study confirms the results found in previous studies.(2,3) However, despite providing important confirmatory data, Cantor et al1 appear to reach conclusions that are not supported by their own data. Whereas the 2 previous studies(2,3) conclude that BP is a prodrug that is hydrolyzed to its free acid to account for its ocular hypotensive effect by activation of known FP prostanoid receptors, the current publication1 surprisingly concludes that BP is not a prodrug and acts directly as an amide to reduce intraocular pressure (IOP).

The clinical studies cited above(1-3) are not the only ones that have demonstrated the hydrolysis of BP in ocular tissues. Previous studies have demonstrated its hydrolysis in vitro in rabbit, bovine, and human ocular tissues(4-7) and after topical application in vivo in rabbit and monkey ocular tissues.(8) The hydrolysis of BP to produce sufficient concentrations of its very potent free acid, a well-described FP receptor agonist, provides clear evidence of its prodrug properties. Studies in FP receptor knockout mice have clearly demonstrated the importance of FP receptors for effective IOP reduction after topical application of FP receptor agonists, including BP.(9-12)

The 3 clinical studies1-3 provide very consistent data. Each demonstrates equal or higher levels of the free acid than the intact amide of BP in aqueous humor. Each demonstrates peak levels occurring within the first few hours after topical application of BP, with lower levels afterwards. Peak concentrations of the free acid were 35 nM,3 22 nM,2 and 7 nM1 in each of the 3 studies. After topical application of latanoprost (LP), Cantor et al1 demonstrated a free acid concentration in aqueous of 41 nM at 3 hours, which is less that half of the 100 nM concentration found in a previous study.13 Therefore, when assessing the 4 clinical studies1-3,(13) which evaluated the hydrolysis of LP and/or BP, the lowest concentrations (2 to 5-fold less than the other studies) of the free acids of either LP or BP were consistently demonstrated by Cantor et al.1 Cantor et al1 suggest that the lower levels might be partially explained by single dose administration of BP or LP in their study, compared with multiple dosing in other studies. However, another clinical study also involved single dose administration and yet found approximately 2-fold higher concentrations of LP acid.(13)

The key and unambiguous observation that we believe is critical is that, no matter what values were obtained in each and every study, all of the concentrations, including the lower concentrations of the free acid of either LP or BP found by Cantor et al,1 are sufficiently high enough to account for their activity at FP prostanoid receptors. At 24 hours after LP, the aqueous humor concentration of the free acid was found to be well less than 1 nM,13 demonstrating that very low aqueous concentrations are found during periods of substantial IOP reduction. Therefore, the lower limits of quantitation of 1.3 nM for BP acid and 2.6 nM for LP acid in the current study1 apparently are not sensitive enough to detect substantial, clinically significant aqueous concentrations of the free acids of these prodrugs.

Although lower than the free acid concentrations, the relatively high concentrations of the amide of BP in the aqueous1-3 compared with nondetectable levels of the LP ester is hardly an adequate criterion to support the hypothesis that BP is not a prodrug. These data instead demonstrate that BP amide is an inefficient prodrug compared with LP ester simply because the ester bond is more labile than the amide bond. BP is topically applied at 6 times the concentration of LP, but, unlike LP, is not completely hydrolyzed. 1-3,13 Despite its 6-fold higher concentration, BP yields peak free acid concentrations in the aqueous 3 to 6 times lower than LP acid, 1-3,13 clearly demonstrating the inefficiency of hydrolysis of the amide compared with ester prodrug. Despite these lower concentrations of the BP free acid, they appear to be sufficient to fully account for the effect of BP. Studies have repeatedly demonstrated that the free acid of BP is 3 to 10 times more potent at the FP receptor than the free acid of LP.(14-23) Therefore, the 3 to 6-fold lower concentrations of BP, compared with LP, acid in aqueous still can account for similar activation of FP receptors when their differences in potency are considered.

Despite the overwhelming evidence for greater potency of the acid of BP compared with LP, (14-23) Cantor et al1 claim that the acids are equally potent and cite a single published study(19) using trabecular meshwork cells from a limited number of donor eyes to support their claim. Cantor et al fail to acknowledge that BP acid exhibits a potency (EC50) of 2.8 to 3.8 nM in numerous cell-types derived from several different species (e.g. human ciliary muscle cells, mouse fibroblast, and rat smooth muscle cells)(18) such that even the amount of BP acid they detected (7 nM) would be sufficient to occupy and stimulate many of the FP receptors in the target tissues. In more comprehensive studies using trabecular meshwork cells derived from numerous donor eyes,(18) BP acid still exhibits a relatively high potency (EC50 = 26 ± 10 nM) at the FP receptor that could help account for the observations of Cantor et al.(1) By citing and concentrating on only a single reference as opposed to the many other published studies, Cantor et al1 appear to bias their interpretation of their data.

We fully agree that drugs might reach target tissues via routes independent of aqueous humor. However, in the case of prostaglandin analogs, including both LP and BP, drug concentrations measured in the aqueous will leave the eye via trabecular or uveoscleral outflow pathways, thereby providing active drug to these tissues. These drugs also might enter these outflow tissues by other routes, and also might be hydrolyzed to their active free acids either along these alternative routes or after they arrive at their target tissue. Therefore, the possibility of alternative routes of delivery of these drugs to target tissues does not negate their action as prodrugs.

While the manufacturer of BP has repeatedly tried to present data to support the hypothesis that BP’s mechanism of action does not rely on FP receptor agonism of the free acid but rather is due to intrinsic receptor occupancy of the amide, their arguments fail to be convincing for several reasons. First, as previously reported, the presence of FP receptors are essential for BP’s hypotensive action as demonstrated by experiments in FP knockout mice.(10,11) More importantly, there has not been, to the best of our knowledge, any putative receptor that has been adequately identified that can explain the actions of BP at a unique non-FP receptor. The elusive, mystery receptor in question has never been cloned or characterized by receptor binding kinetics. In short, BP’s hypotensive action appears to require FP receptor agonism that occurs following the hydrolysis of the prodrug that liberates the free acid that then activates the classic FP receptors in the target tissues. The shared characteristics of BP with other prostaglandin analogs, including the side effects of iris color darkening and eyelash changes, also would argue that BP exerts its biological actions by activating the FP receptor. Even if “prostamide” receptors were demonstrated to exist in the anterior uvea, the IOP effect of BP still can be more reasonably explained by its ability to activate FP prostanoid receptors following its demonstrated hydrolysis to its potent free acid. In conclusion, we agree with the data presented by Cantor et al1 finding substantial, albeit somewhat lower,(2,3,13) concentrations of the free acid of BP or LP in aqueous after topical application in humans. However, we strongly disagree with their conclusions. Cantor et al,(1) like others,(2,3,13) have confirmed that BP is an inefficient prodrug that is hydrolyzed to its free acid to activate well-described FP prostanoid receptors resulting in IOP reduction. Postulation of the existence of enigmatic “prostamide” or yet to be identified unknown receptors is not necessary or warranted.

Carl B. Camras, M.D., Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska

Najam A. Sharif, Ph.D., Molecular Pharmacology Unit, Alcon Research, Ltd. Fort Worth, TX

Martin B. Wax, M.D., Research and Development, Ophthalmology Discovery Research, Alcon Laboratories, Fort Worth, TX; Department of Ophthalmology and Visual Sciences, University of Texas Southwestern Medical School, Dallas, TX

Johan Stjernschantz, M.D., Ph.D., Department of Neuroscience, Uppsala University Biomedical Center, Uppsala, Sweden

Supported in part by an unrestricted grant from Research to Prevent Blindness, Inc., New York, NY. Dr. Camras was a consultant to Pfizer Ophthalmics. Drs. Sharif and Wax are employees of Alcon Laboratories. Dr. Stjernschantz was an employee of Pharmacia Ophthalmics.

Correspondence to: Carl B. Camras, M.D., Department of Ophthalmology and Visual Sciences, 985540 Nebraska Medical Center, Omaha, NE 68198-5540

References:

1 Cantor LB, Hoop J, WuDunn D, et al. Levels of bimatoprost acid in the aqueous humour after bimatoprost treatment of patients with cataract. Br J Ophthalmol 2007;91:629-32.

2 Camras CB, Toris CB, Sjoquist B, et al. Detection of the free acid of bimatoprost in aqueous humor samples from human eyes treated with bimatoprost before cataract surgery. Ophthalmology Same 2004;111:2193-8.

3 Dahlin DC, Craven ER, Moster M, et al. Human aqueous humor concentrations of bimatoprost and bimatoprost free acid following topical ocular dosing of Lumigan (bimatoprost (17-phenyl-trinor-PGF2a) 0.03% ophthalmic solution) [abstract]. Invest Ophthalmol Vis Sci 2004;45:ARVO E-Abstract 2096.

4 Maxey KM, Johnson JL, LaBrecque J. The hydrolysis of bimatoprost in corneal tissue generates a potent prostanoid FP receptor agonist. Surv Ophthalmol 2002;47(Suppl 1):S34-S40.

5 Davies SS, Ju WK, Neufeld AH, et al. Hydrolysis of bimatoprost (Lumigan) to its free acid by ocular tissue in vitro. J Ocul Pharmacol Ther 2003;19:45-54.

6 Hellberg MR, Ke TL, Haggard K, et al. The hydrolysis of the prostaglandin analog prodrug bimatoprost to 17-phenyl-trinor PGF2a by human and rabbit ocular tissue . J Ocul Pharmacol Ther 2003;19:97-103.

7 Kriatchko A, Zhan G, Cheruvu N, et al. In vitro transport and hydrolysis of bimatoprost in bovine cornea [abstract]. ARVO 2003;B81.

8 Dahlin DC, Bergamini MVW, Curtis MA, et al. Bimatoprost hydrolysis to 17-phenyl PGF2a by rabbit and monkey ocular tissues, in vivo [abstract]. Invest Ophthalmol Vis Sci 2002;43:ARVO E-Abstract 4109.

9 Ota T, Aihara M, Saeki T, et al. The IOP-lowering effects and mechanism of action of tafluprost in prostanoid receptor-deficient mice. Br J Ophthalmol 2007;91:673-6.

10 Crowston JG, Lindsey JD, Morris CA, et al. Effect of bimatoprost on intraocular pressure in prostaglandin FP receptor knockout mice. Invest Ophthalmol Vis Sci 2005;46:4571-7.

11 Ota T, Aihara M, Narumiya S, et al. The effects of prostaglandin analogues on IOP in prostanoid FP-receptor-deficient mice. Invest Ophthalmol Vis Sci 2005;46:4159-63.

12 Crowston JG, Lindsey JD, Aihara M, et al. Effect of latanoprost on intraocular pressure in mice lacking the prostaglandin FP receptor. Invest Ophthalmol Vis Sci 2004;45:3555-9.

13 Sjöquist B, Stjernschantz J. Ocular and systemic pharmacokinetics of latanoprost in humans. Surv Ophthalmol 2002;47(Suppl 1):S6-S12.

14 Resul B, Stjernschantz J, Selén G, et al. Structure-activity relationships and receptor profiles of some ocular hypotensive prostanoids. Surv Ophthalmol 1997;41(Suppl 2):S47-S52.

15 Stjernschantz JW. From PGF2a-isopropyl ester to latanoprost: a review of the development of Xalatan: the Proctor Lecture. Invest Ophthalmol Vis Sci 2001;42:1134-45.

16 Sharif NA, Williams GW, Kelly CR. Bimatoprost and its free acid are prostaglandin FP receptor agonists. Eur J Pharmacol 2001;432:211-3.

17 Sharif NA, Kelly CR, Crider JY, et al. Ocular hypotensive FP prostaglandin (PG) analogs: PG receptor subtype binding affinities and selectivities, and agonist potencies at FP and other PG receptors in cultured cells. J Ocul Pharmacol Ther 2003;19:501-15.

18 Sharif NA, Crider JY, Husain S, et al. Human ciliary muscle cell responses to FP-class prostaglandin analogs: phosphoinositide hydrolysis, intracellular Ca2+ mobilization and MAP kinase activation. J Ocul Pharmacol Ther 2003;19:437-55.

19 Sharif NA, Kelly CR, Crider JY. Human trabecular meshwork cell responses induced by bimatoprost, travoprost, unoprostone, and other FP prostaglandin receptor agonist analogues. Invest Ophthalmol Vis Sci 2003;44:715-21.

20 Sharif NA, Kelly CR, Williams GW. Bimatoprost (Lumigan®) is an agonist at the cloned human ocular FP prostaglandin receptor: real-time FLIPR-based intracelluar Ca2+ mobilization studies . Prostaglandins Leukot Essent Fatty Acids 2003;68:27-33.

21 Sharif NA, Kelly CR, Crider JY. Agonist activity of bimatoprost, travoprost, latanoprost, unoprostone isopropyl ester and other prostaglandin analogs at the cloned human ciliary body FP prostaglandin receptor. J Ocul Pharmacol Ther 2002;18:313-24.

22 Kelly CR, Williams GW, Sharif NA. Real-time intracellular Ca2+ mobilization by travoprost acid, bimatoprost, unoprostone, and other analogs via endogenous mouse, rat, and cloned human FP prostaglandin receptors. J Pharmacol Exp Ther 2003;304:238-45.

23 Stjernschantz J, Albert D, Hu D, et al. Mechanism and clinical significance of prostaglandin-induced iris pigmentation. Surv Ophthalmol 2002;47 (suppl 1):S162-S175.