Dear Editor

According to the Canadian Community Health Survey, approximately 82% of the population of seniors aged 65 – 80+ (3,000,000 seniors) reported having vision problems in Canada. [1] Cataracts are the leading cause of vision impairment among seniors. Between the period of 1994 – 2003, proportions of seniors with cataracts rose from 14% – 20% with populations aged 75 and over accounting for higher percentages. [1]

Cataract surgery is among non-emergency surgical procedures with highest wait times in Canada. There was a 32% increase in cataract surgeries over 5 years between 1997/1998 and 2002/2003. In British Columbia, there were 11,816 patients waiting for surgery and 7897 patients completed in the 3 months from 31 August 2006 to 31 October 2006. [2] In addition to being a barometer of accessibility to health care services, cataract wait times are also a determinant of patient satisfaction which in turn is correlated with increased health-related quality of life [3] and possibly decreased injury risk. [4]

Delay of care is a persistent and undesirable feature of current health care systems. [5] Waits and delays plague health care systems worldwide, and wait times for most specialists exceed those for primary care practices. [6] From clinical perspective delay in necessary treatment due to surgical wait lists is a major concern. [7] Establishing a clinically appropriate time that patients can safely wait for the operation is generally perceived as a method to prevent adverse outcomes of delay. [8]

The cost effectiveness of cataract surgery has been well-established. [9, 10] In fact, modern techniques used for cataract surgery today result in rapid visual improvement with 50% of patients experiencing good vision by 24 hours and 96-99% experiencing good vision by 4 weeks. [11] Evidence supports cataract surgery among older drivers in producing significant improvements in driving performance (best predicted by the concomitant improvement in contrast sensitivity), subsequent crash rates half that of older drivers with cataracts who opted not to have surgery, and self-reported improved visual function and distance estimation. [4]

References

(1). Millar, W. J. Vision problems among seniors. Health Reports. 2004; 16; 45-49.

(2). BC Ministry of Health (2006). Surgical Wait Times: Cataract Surgery in BC. [Online]. Available at URL: http://www.swl.hlth.gov.bc.ca/swl/swl_db/swl.WaitlistPkg.GetHospitalListBySurgSpecNLF?IEvent=27

(3). Conner-Spady, B.L., Sanmugasunderam, S., Courtright, P., McGurran, J.J., & Noseworthy, T.W. Determinants of patient satisfaction with cataract surgery and length of time on the waiting list. British Journal of Ophthamology. 2004; 88; 1305-1309.

(4). Owsley, C., McGwin, G., Sloane, M., Wells, J., Stalvey, B.T., Gauthreaux, S. Impact of cataract surgery on motor vehicle crash involvement by older adults. JAMA. 2002; 21; 288(7):841-9.

(5). Hodge, W., Horsley, T., Albiani, D., Baryla, J., Belliveau, M., Buhrmann, R., O'Connor, M., Blair, J., Lowcock, E. The consequences of waiting for cataract surgery: a systematic review. CMAJ. 2007 176: 1285 - 1290.

(6). Murray, M.F. Improving access to specialty care. Jt Comm J Qual Patient Saf. 2007; 33(3):125-35.

(7). Sobolev, B., Mercer, D., Brown, P., FitzGerald, M., Jalink, D., Shaw, R. Risk of emergency admission while awaiting elective cholecystectomy. CMAJ. 2003; 169; 662–665. (8). MacCormick, A.D., Collecutt, W.G., Parry, B.R. Prioritizing patients for elective surgery: a systematic review. ANZ J Surg. 2003; 73; 633–642. doi: 10.1046/j.1445-2197.2003.02605.x.

(9). Laidlaw, D.A.H., Harrad, R.A., Hopper, C.D., Whitaker, A., Donovan, J.L., Brookes, S.T., Marsh, G.W., Peters, T.J., & Sparrow, J. M. Randomised trial of effectiveness of second eye cataract surgery. Lancet. 1998; 352; 925-929.

(10) Sach, T.H., Foss, A., Gregson, R., Zaman, A., Osborn, F., Masud, T., Harwood, R.H. Falls and health status in elderly women following first eye cataract surgery: an economic evaluation conducted alongside a randomised controlled trial. Br J Ophthalmol. 2007 Jun 21; [Epub ahead of print]

(11). Harwood, R. H., Foss, A. J. E., Osborn, F., Gregson, R.M., Zaman, A., & Masud, T. Falls and health status in elderly women following first eye cataract surgery: A randomized controlled trial. British Journal of Ophthamology. 2005; 85; 53-59.

Dear Editor

We read with interest the paper by Keenan [ref 1] and colleagues.

We recently studied the 2005/06 Hospital Episode Statistics (HES) data for cataract and have reached similar conclusions. We agree with the authors’ observations that as NHS cataract surgery rates (CSRs) in England have increased significantly in recent years, several questions are now raised. What is the appropriate CSR in a developed market economy? Is the observed geographical variation in CSR in England a marker of increased incidence and requirement for surgery in some ‘high activity’ regions or a marker for overprovision? Conversely, in relation to lower activity regions, are there data set issues from the use of an administrative database such as the HES, or are there genuine clinical differences or variations in the organisational provision of cataract care? Private CSR rates are not considered as HES returns are restricted to NHS care. In our consideration of the most up to date HES returns there are some data problems -due to coding issues- at some hospital Trusts (details on request). Observational studies of cataract surgical practice and population studies of regional prevalence of cataract are now needed. An electronic clinical database -a cataract national dataset- is the most pragmatic method of capturing such data in a timely fashion. Such an e-tool would track changing thresholds for referral for cataract surgery and importantly, consider outcomes for patients at a national level. Such an initiative is technically possible, is strongly supported by the Royal College of Ophthalmologists and now requires government support. With the UK Government’s commitment to developing an electronic patient care record, the deployment of a national electronic cataract dataset would greatly assist in our better understanding of these questions.

Care of NHS cataract patients has improved as a result of better technology and improved access to care, much of which followed the Action on Cataract (AoC) initiative. [2] A recent quality improvement report in relation to NHS cataract care in Scotland has been compelling. [3] However ophthalmologists and commissioners of ophthalmic care should not become complacent. Pressures on eye care services for the future are likely to be significant; as a result of the aging population, lower clinical thresholds for safe treatment for cataract, the introduction of new treatments for patients with age-related maculopathy and other conditions.

Cataract surgery is a highly effective procedure which provides rapid improvement in vision related, as well as non-vision related, outcomes as well as being very cost effective. [4] Benefits to patients are lifelong. The principal causal factor of adult cataract is ageing and demand for services for cataract and other diseases of the aging eye are expected to increase as the UK population ages. The indications for surgery as recommended in the consensus guidelines from the College, simply stated, are: failing vision attributable to lens opacity despite optimal optical correction or ocular co-morbidity and patient willingness and fitness to undergo cataract surgery. The last issue is not problematic as surgery is almost always carried out under day care and local anaesthesia. There is no evidence, that we are aware of, to suggest that patients are having ‘inappropriate’ cataract surgery in the UK

We are aware some Primary Care Trusts (PCTs) are attempting to ‘demand manage’ cataract surgery to certain thresholds of patient visual impairment. Such decisions, if simply based on Snellen visual acuity levels, are likely to disadvantage elderly patients. [5] Attempts to include chronological age as a factor in healthcare policy are likely to be insensitive. [6] Some have suggested that access to surgery could be determined by an assessment of a range of a patient’s visual symptoms and disability, rather than a simple measurement of monocular visual acuity. [7] We support the concept that decision to operate for cataract remains a matter of balanced clinical judgment and consensus reaching with the individual patient. The World Health Organization recommended 3,000 cataract operations per million residents as the minimum to eliminate blindness from cataract and recommended 3,500 per million for established market economics for the year 2000. [8] This latter target is being attained across the English NHS.

References

1. Keenan T, Rosen P, Yeates D, Goldacre M. Time trends and geographical variation in cataract surgery rates in England: study of surgical workload Br J Ophthalmol 2007; EP. doi: 10.1136/bjo.2006.108977

2. Department of Health. Action on cataracts – Good Practice Guidance. NHS Executive. Feb 2000 http://www.dh.gov.uk/assetRoot/04/01/45/14/04014514.pdf

3. Tey A, Grant B, Harbison D, Sutherland S, Kearns P, Sanders R. Redesign and Modernisation of an NHS cataract service (Fife 1997-2004): multifaceted approach. BMJ. 2007; 334: 148-152

4. Walker JG, Anstey KJ, Hennessy MP, Lord SR, von Sanden C. The impact of cataract surgery on visual functioning, vision-related disability and psychological distress: a randomized controlled trial. Clin Experiment Ophthalmol. 34, 734-42 (2006)

5. Westcott, M C, Tuft, S J, Minassian, D C Effect of age on visual outcome following cataract extraction Br J Ophthalmol 2000 84: 1380-1382

6. Beare N, Dandona L. Cataract surgery in very elderly patients BMJ 2001;323:455.

7. Crabtree HL, Hildreth AJ, O'Connell JE, et al. Measuring visual symptoms in British cataract patients: the Cataract Symptom Scale. Br J Ophthalmol 1999;83:519-523

8. World Health Organisation Global Initiative for the Elimination of Avoidable Blindness. Geneva, Switzerland: World Health Organization; 2000. WHO/PBL/97.61 Rev 2.

Dear Editor

We read with interest the letter by Decock et al regarding the rare congenital ocular malformation (orbital cyst and bilateral colobomatous microphthalmos). [1]

The female child reported was the product of a mother with documented vitamin A deficiency (VAD) during the second trimester of pregnancy after previous gastric bypass surgery for morbid obesity. The authors do not report the symptoms or biochemical evidence of VAD in this particular mother nor the treatment she received but presumably it is likely that she had subclinical VAD during the first trimester when the foetal fissure may be disturbed causing coloboma.

This has striking parallels with a cases series of children with ocular coloboma from South India reported previously by our group. In that study maternal night blindess presumable from VAD was reported in 16% of mothers of children with coloboma compared with 5% rate for the normal local population of mothers. Studies by the Indian Institute of Nutrition of the same population showed that 50% of mothers have biochemical evidence of VAD during pregnancy mostly asymptomatic and subclinical. [2]

We have previously reported epidemiological and laboratory evidence to support a hypothesis that there may be a genetic (recessive) predisposition to the teratogenic effect of mild to moderate maternal vitamin A deficiency in pregnancy. This may explain the higher prevalence of colobomatous malformations in certain Asian countries where maternal vitamin A deficiency is common and consanguineous marriages popular, the low risk to siblings, and birth-order effect with increased frequency of colobomas in higher birth-orders. Mutations in a gene involved in cellular access to vitamin A that normally protects the mother or the embryo from natural variation in dietary vitamin A intake could render that individual intolerant of conditions of vitamin A deficiency. [3]

If such a gene-environment interaction is shown to be true there are public health implications for the prevention of blindness from anomalies akin to the research showing benefits of folic acid supplementation in prevention of neural tube defects. However, this form of intervention would be much more difficult with vitamin A which is itself a powerful teratogen if present in excess.

References

1. Decock, C. E., Breusegem, C. M., Van Aken, E. H, and Leroy, B. P. High beta-trace protein concentration in the fluid of an orbital cyst associated with bilateral colobomatous microphthalmos. Br.J Ophthalmol. 91(6), 836-837. 2007.

2. Hornby SJ, Ward SJ, Gilbert CE, Dandona L, Foster A, Jones RB. Environmental risk factors in the aetiology of congenital malformations of the eye in children in South India. Arch.Trop.Paed. 2002;22:67-77.

3. Hornby, S. J., Gilbert, C. E., and Ward, S. J. Eye birth defects in humans may be caused by a recessively-inherited genetic predisposition to the effects of maternal vitamin A deficiency during pregnancy. Med Sci Monit 9(11), HY23-26. 2003.

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.