Lack of evidence for a link between latanoprost use and malignant melanoma: an analysis of safety databases and a review of the literature

AIM

To determine if there is an association between the use of latanoprost ophthalmic solution and malignant melanoma and to assess the evidence of a plausible biological mechanism.

METHODS

Two safety databases were reviewed: one representing all latanoprost (n=24) and fixed-combination latanoprost/timolol (n=16) clinical trials conducted from November 1992 through November 2007 and a global safety database of all spontaneous non-trial-related clinical reports spanning 13 and 9 years for latanoprost and for latanoprost/timolol, respectively. A systematic PubMed search for studies evaluating potential mechanisms was conducted.

RESULTS

Amongst 12,880 latanoprost-treated subjects in clinical trials, no reported cases of ocular melanoma and three cases of cutaneous melanoma were identified. Of 19,940 cases recorded in the global safety database, 22 reports of ocular/cutaneous neoplasms were identified. Of these neoplasms, 11 were ocular and six were cutaneous melanomas. Possible association with latanoprost use could not be excluded in three ocular and one periorbital report. In vitro and in vivo data were consistent with a mechanism whereby the increased iris pigmentation results from stimulation of melanin synthesis by induction of tyrosinase transcription without increasing mitotic activity.

CONCLUSION

There is no evidence at present that establishes a link between latanoprost use and either ocular or cutaneous melanoma.

[The pathology of ocular syndromes caused by toxicity]

Ocular adnexa represent a complex system of delicate organs and functions which are the target of varied side effects. Most involve more than one component at a time, however landmark signs and symptoms can be outlined. Dry eye leads the list. The aqueous production of the tear film can be decreased by certain psychotherapeutic agents (especially the older ones), while the phospholipidic component, produced by the Meibomian glands, can be markedly affected by retinoids. On the other hand, cytostatic drugs like Docetaxel (and 5-FU at a lesser degree) frequently induce canalicular stenosis, resulting in epiphora. Amongst a long list of substances, diphosphonates used in the treatment of osteoporosis and phosphodiesterase-5 inhibitors used in erection deficiencies induce conjunctival irritation, either directly or by contiguity. Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis represent the most severe toxic insult to the mucosae. A recent chemotherapeutic agent, Imatinib, induces fluctuant palpebral edema in a majority of patients. Despite being applied topically, prostaglandin analogs exert a profound effect onto the cellular physiology of the eyelash and the ocular and palpebral melanocyte. Indirectly, immunosuppressive agents used in graft rejection control have been associated with the emergence of secondary neoplasia, mainly lymphoma, of which the orbit is a rare but possible location. Chronic administration of steroid drugs leads to hypertrophy of the orbital fat and proptosis.

Inhibition of urethane-induced carcinogenicity in cyp2e1-/- in comparison to cyp2e1+/+ mice

Urethane is an established animal carcinogen and has been classified as "reasonably anticipated to be a human carcinogen." Until recently, urethane metabolism via esterase was considered the main metabolic pathway of this chemical. However, recent studies in this laboratory showed that CYP2E1, and not esterase, is the primary enzyme responsible for urethane oxidation. Subsequent studies demonstrated significant inhibition of urethane-induced genotoxicity and cell proliferation in Cyp2e1-/- compared to Cyp2e1+/+ mice. Using Cyp2e1-/- mice, current studies were undertaken to assess the relationships between urethane metabolism and carcinogenicity. Urethane was administered via gavage at 1, 10, or 100 mg/kg/day, 5 days/week, for 6 weeks. Animals were kept without chemical administration for 7 months after which they were euthanized, and urethane carcinogenicity was assessed. Microscopic examination showed a significant reduction in the incidences of liver hemangiomas and hemangiosarcomas in Cyp2e1-/- compared to Cyp2e+/+ mice. Lung nodules increased in a dose-dependent manner and were less prevalent in Cyp2e1-/- compared to Cyp2e+/+ mice. Microscopic alterations included bronchoalveolar adenomas, and in one Cyp2e1+/+ mouse treated with 100 mg/kg urethane, a bronchoalveolar carcinoma was diagnosed. Significant reduction in the incidence of adenomas and the number of adenomas/lung were observed in Cyp2e1-/- compared to Cyp2e1+/+ mice. In the Harderian gland, the incidences of hyperplasia and adenomas were significantly lower in Cyp2e1-/- compared to Cyp2e+/+ mice at the 10 mg/kg dose, with no significant differences observed at the high or low doses. In conclusion, this work demonstrated a significant reduction of urethane-induced carcinogenicity in Cyp2e1-/- compared to Cyp2e1+/+ mice and proved that CYP2E1-mediated oxidation plays an essential role in urethane-induced carcinogenicity.

Evaluation of the carcinogenic potential of clofibrate in the rasH2 mouse

The purpose of the study was to support of the International Life Sciences Institute (ILSI) alternative carcinogenicity models initiative to evaluate the carcinogenic potential of the nongenotoxic carcinogen, clofibrate, a peroxisome proliferator-activated receptor (PPAR) alpha agonist, following oral administration to rasH2 mice. Peroxisome proliferators are one of the most widely studied of the nongenotoxic carcinogens and have diverse industrial and therapeutic uses (Gonzalez et al. J. Nat. Cancer Inst. 90: 1702-1709, 1998); however, the nongenotoxic mechanism of carcinogenicity is currently unknown. Male mice were administered doses of clofibrate at 50, 100, or 200 mg/kg/day and female mice were administered doses of 50, 150, or 250 mg/kg/day by oral gavage at 10 ml/kg for 27 weeks. In addition, rasH2 male and female mice were treated with N-nitroso-N-methylurea (NMU). Nontransgenic male and female mice were treated with 200 and 250 mg/kg/day, respectively, of clofibrate. The NMU-treated mice were given a single intraperitoneal dose of 75 mg/kg, which was followed by a 90-day observation period; all others were sacrificed after 6 months of daily dosing. Hepatocellular neoplasms were observed in clofibrate-treated rasH2 male mice after 6 months of treatment but not in nontransgenic males or females. Clofibrate treatment (250 mg/kg/day) of female rasH2 mice was associated with a slight increase in the incidence of various neoplasms (harderian gland, lungs, skin, spleen, tail, thymus, and uterus) compared with untreated transgenic mice and with similarly treated nontransgenic mice. Non-neoplastic changes were found in the liver of transgenic and nontransgenic mice of both sexes and in the kidneys of male mice. NMU produced findings are consistent with previous studies. The data suggest that the rasH2 mice are a good model for testing epigenetic carcinogens in a shorter timeframe than conventional mouse carcinogenicity bioassays.

Carcinogenicity of glycidol in F344 rats and B6C3F1 mice

Glycidol, a simple aliphatic epoxide, was administered by gavage in water to groups of male and female F344/N rats and B6C3F1 mice. Rats received 0, 37.5 or 75 mg kg-1 and mice received 0, 25 or 50 mg kg-1 daily, 5 days per week for 2 years. Exposure to glycidol was associated with dose-related increases in the incidences of neoplasms in numerous tissues in both rats and mice. Survival of rats that received glycidol was markedly reduced compared to the control because of the early induction of neoplastic disease. In male rats, mesothelioma arising in the tunica vaginalis and frequently metastasizing to the peritoneum were considered the major cause of early death. Early deaths in female rats were associated with mammary gland neoplasms. Survival of female mice that received 50 mg kg-1 was lower than the control after week 101 due primarily to euthanasia of moribund animals with mammary gland neoplasms. Survival of male mice and female mice that received 25 mg kg-1 was comparable to the control. In mice, exposure to glycidol was associated with increased incidences of neoplasms of the harderian gland in males and females, the forestomach in males and the mammary gland in females.

Increased susceptibility to ultraviolet-B and carcinogens of mice lacking the DNA excision repair gene XPA

Xeroderma pigmentosum patients with a defect in the nucleotide-excision repair gene XPA are characterized by, for example, a > 1,000-fold higher risk of developing sunlight-induced skin cancer. Nucleotide-excision repair (NER) is involved in the removal of a wide spectrum of DNA lesions. The XPA protein functions in a pre-incision step, the recognition of DNA damage. To permit the functional analysis of the XPA gene in vivo, we have generated XPA-deficient mice by gene targeting in embryonic stem cells. The XPA-/-mice appear normal, at least until the age of 13 months. XPA-/-mice are highly susceptible to ultraviolet (UV)-B-induced skin and eye tumours and to 7,12-dimethylbenz[a]anthracene (DMBA)-induced skin tumours. We conclude that the XPA-deficient mice strongly mimic the phenotype of humans with xeroderma pigmentosum.

Decreases in spontaneous tumors in rats and mice after treatment with amphetamine

Toxicology and carcinogenesis studies of dl-amphetamine sulfate, a drug used in the treatment of weight control, narcolepsy, and behavioral syndromes in children, were performed in F344/N rats and B6C3F1 mice. In these studies, amphetamine was administered for 2 years at doses of 0, 20, or 100 ppm in the feed to groups of 50 animals/dose/sex/species. The average amount of amphetamine consumed per day was estimated to be 1 or 5 mg/kg for low or high dose rats, 4 or 30 mg/kg for low or high dose male mice, and 3 or 19 mg/kg for low or high dose female mice. Survival was similar in dosed and control groups. The most notable effect of long-term treatment with this drug was the reduction of body weight in comparison to controls, and reduction in spontaneous tumors including pheochromocytomas of the adrenal gland in male rats, fibroadenomas of the mammary gland in female rats, adenomas of the anterior pituitary gland in male and female rats and female mice, endometrial stromal polyps of the uterus of female rats, adenomas or carcinomas of the liver in male and female mice, adenomas of the Harderian gland in male and female mice, and adenomas or carcinomas of the lung in male and female mice. Decreases in spontaneous tumors have previously been seen in 2-year rodent studies in groups of animals that have a reduced body weight in comparison to controls, but the spectrum of reduction in spontaneous neoplasms after treatment with amphetamine is broader than has previously been observed.

Immortalized retinal neurons derived from SV40 T-antigen-induced tumors in transgenic mice

Immortalized retinal neurons have been established in tissue culture from retinal tumors arising in transgenic mice. The mice carry the SV40 T-antigen under the control of 5' flanking sequences from the human phenylethanolamine N-methyltransferase (PNMT) gene in order to target oncogene expression to adrenergic cell types. The retinal cultures contain a proliferation population of T-antigen-positive cells with a neuronal morphology that includes formation of extensive neuritic processes. We identified the cells as amacrine-derived neurons by immunofluorescence using the cell-specific monoclonal antibodies VC1.1 and HPC-1. The cells also express all three neurofilament subunits and GAP-43. These results indicate that CNS neurons can be transformed in transgenic animals to generate cultured cells with many properties of mature neurons.

Induction of ocular tumor by nickel subsulfide in the Japanese common newt, Cynops pyrrhogaster

Chemically induced ocular tumors in amphibians have not been reported previously. In the present study, nickel subsulfide (Ni3S2) was administered to lentectomized Japanese common newts, Cynops pyrrhogaster, by a single injection into the posterior chamber of the right eye (40-100 micrograms Ni3S2/newt). Control newts received a similar injection of vehicle. Malignant melanoma-like tumors developed in the injected eyes of seven of eight Ni3S2-treated newts at 9 months (versus none of six controls). Tumor cells occupied the entire globe and invaded the surrounding tissues. At 11 months melanoma-like cells in treated eyes exhibited by electron microscopy deeply invaginated nuclei and various degrees of pigmentation. Some cells even contained crystalline Ni3S2 in the cytoplasm at 11 months. Mitotic figures were rare in treated eyes at 9 months but were often found in them at 3 months. In one of the seven tumor-bearing eyes at 9 months, a prominent metaplastic cartilage was observed within the globe. A regenerated lens was noted at 9 months in a Ni3S2-treated eye which escaped tumor induction. The site of tumor origin was assumed to be the iris, because the aberrantly proliferating cell population was predominantly found in the root of the iris in treated eyes at 3 months.

Intraocular neoplasms induced by methylazoxymethanol acetate in Japanese medaka (Oryzias latipes)

Intraocular neoplasms developed in the Japanese medaka, a small fish species, following a single brief exposure to methylazoxymethanol acetate [(MAM-Ac) CAS: 592-62-1]. Specimens 6-10 days old were exposed to doses of MAM-Ac up to 100 mg/liter for 2 hours and then transferred to carcinogen-free water for "grow-out." Of 218 exposed fish examined, 98 (45%) had neoplastic lesions in various stages of development. Of those exposed to 30 mg/liter or more, 57% had the lesions. No lesions were found in eyes or other tissues of 95 control specimens. Early and advanced neoplastic lesions were recognized. Early lesions were characterized by complexes of neoplastic retinal epithelium and tubes that consisted of cells of the sensory retina. Areas of mitotically active, heterogeneous cells associated with such complexes gave rise to advanced neoplasms. We considered the advanced neoplasms to be medulloepitheliomas, which differentiated into three principal cellular patterns: 1) solid masses of unpigmented cells, which frequently showed photoreceptor differentiation as well as ductular formation; 2) heavily pigmented cuboidal to columnar cells resembling retinal epithelium that formed adenomatous patterns; and 3) teratoid medulloepitheliomas. Teratoid medulloepitheliomas, which we considered the most advanced and malignant lesions, consisted of heterogeneous, highly mitotic, invasive cells and contained heteroplastic elements including striated muscle, undifferentiated mesenchymal tissues, and hyaline cartilage. We suggest that MAM-Ac induces hyperplasia of retinal cells followed by establishment of aberrant growth zones containing miscoded cells that give rise to medulloepitheliomas.

Development of aquarium fish models for environmental carcinogenesis: tumor induction in seven species

For small fish species to be utilized as models for carcinogenicity testing they should be capable of developing neoplasms, preferably in multiple tissues, when exposed to known carcinogens. Seven species of small fish were exposed to methylazoxymethanol acetate (MAM-Ac) and tumor development was monitored. Specimens 6-10 days old were exposed to nominal concentrations of MAM-Ac up to 100 mg 1(-1) for 2 h, then transferred to carcinogen-free water. Hepatic neoplasms developed in the Japanese medaka, guppy, sheepshead minnow, Gulf killifish, inland silverside, rivulus, and fathead minnow. Additionally, neoplasms occurred in other organs and tissues of the medaka (retina, various mesenchymal tissues, exocrine pancreas, kidney, and nervous tissue), guppy (mesenchymal tissue, exocrine pancreas, and kidney), and sheepshead minnow (choroid gland, mesenchymal tissues, and nervous tissue). All tumors were diagnosed in specimens within 1 year post-exposure. Early signs of liver tumors appeared in medaka and guppy at about 1 month post-exposure. These studies show that both medaka and guppy would be good models because they appear sensitive to carcinogens, develop tumors in multiple tissues and are easy to breed and maintain. Certain other small fish species also may prove to be good models because of habitat preferences, breeding strategies, or genetic attributes.

Carcinogenicity of dichloroacetylene: an inhalation study

Dichloroacetylene (DCA) is a by-product of the synthesis of certain chlorinated, aliphatic hydrocarbons (e.g., vinylidene chloride). In a long-term carcinogenicity inhalation study, groups of 30 male and 30 female rats and mice were exposed to DCA vapour under the following conditions: mice, group I: 9 p.p.m. 6 h/day, 1 day/week for 12 months; group II: 2 p.p.m. 6 h/day, 1 day/week for 18 months; group III: 2 p.p.m. 6 h/day, 2 days/week for 18 months; rats: 14 p.p.m. 6 h/day, 2 days/week for 18 months; controls: under identical conditions except for addition of DCA to breathing air. The most important result is a striking increase in the formation of kidney cystadenomas of the proximal tubuli in all DCA exposed animals. The median latency time of this tumor varied widely in both mice and rats. Renal cystic adenocarcinomas were found in statistically significant numbers in male mice. In addition to kidney tumors, the development of cystadenomas of the Harderian gland in mice and of liver cholangiomas in rats were highly significant findings. DCA failed to induce liver tumors in mice. Our results confirm that DCA possesses a high carcinogenic potential, pronounced organotropic properties and distinct species differences referring to the number and nature of the induced tumors. The risk associated with handling chlorinated aliphatic hydrocarbons is essentially increased by the decomposition product DCA.

Induction of ocular neoplasms in Fischer rats by intraocular injection of nickel subsulfide

Nickel subsulfide, alpha Ni3S2, was administered to albino Fischer rats by a single injection into the vitreous body of the right eye (0.5 mg alpha Ni3S2/rat, suspended in 20 microliter of NaCl vehicle). Control rats received a similar injection of the vehicle. Malignant tumors developed in the injected eyes of 14/15 alpha Ni3S2-treated rats by 8 months (vs. 0/11 controls, p less than 0.001). Five of the injected eyes of alpha Ni3S2-treated rats contained multiple tumors. The 21 eye tumors that were induced by alpha Ni3S2 included 11 melanomas, four retinoblastomas, three gliomas, and three unclassified malignant neoplasms. Three of the melanomas developed extraocular extensions; one of the melanomas metastasized to lungs and brain. Although the melanomas arose from amelanotic uveal melanocytes, melanosomes were observed in electron micrographs of the tumor cells. This study provides a new experimental model for chemical induction of ocular neoplasms. As a procedure to test the carcinogenicity of nickel compounds, intraocular injection has the advantages of short latency period, high tumor incidence, and ease of tumor detection.

Tissue differentiation and susceptibility to embryonal tumor induction by ethylnitrosourea in the opossum

Opossums (Didelphis virginiana Kerr) exposed to 100 mg ENU/kg in single or incremental doses early in postnatal life developed a spectrum of epithelial and mesenchymal neoplasms including several types of embryonal neoplasms not previously induced in laboratory animals. A correlation was apparent to a varying degree between susceptibility to tumor induction and the state of morphologic maturation of the presumed target tissues at the light microscopic level for embryonal tumors of the eye, kidney, and brain. The susceptibility of the opossum eye to an ENU-induced intraocular teratoid medulloepithelioma extended over the period from 1 to between 3 and 4 weeks of age and was correlated with the differentiation of the apparent target cell, the nonpigmented ciliary epithelium of the pars ciliaris retinae. Induction of nephroblastomas was correlated with the presence in the kidney of stem cells (metanephric blastema) through the period from birth to between 6 and 8 weeks of age. Although susceptibility of the opossum brain to ENU induction of gangliogliomas was correlated with the state of differentiation of the germinal matrix from birth to 56 days of age, induction of these tumors was essentially limited to the 1st week postpartum. No definite correlation between vulnerability to tumor induction and tissue maturation was evident for a tumor of the jaw (ameloblastoma) with presumed origin from embryonic dental remnants. Our results indicated that the opossium early in postnatal life is a useful model for the induction and characterization of certain of the major dysontogenetic tumors, which have been difficult or impossible to reproduce in the traditional laboratory species.

[Detection of the range of cytological changes in tumors induced in rats by nitrosoamines by means of passage in the anterior chamber of the eye]

12 rat liver tumors (high, purely and low differentiated hepatomas and adenocarcinomas), 3 kidney tumors, 1 lymphosarcoma and 1 round cell sarcoma of the uterus were transplanted and passed trough anterior chamber of the allogenic rat eyes. In 8 primary transplants of the liver tumors differentiation traits increased; three of them were morphologically undistinguishable from the normal liver tissue. In some transplants of embryonic liver and kidney displasia was detected. The results obtained give evidence in favor of a very high cytological variability of tumors at the early steps of progression, and indicate that the cultivation in the anterior chamber of the eye may be a proper model for investigation of cytological variability of both malignant and normal tissues.