Phenolphthalein

    The FDA is proposing to ban the over-the-counter (OTC) sale of phenolphthalein, an ingredient widely used in laxatives, because it may pose long-term safety concerns. The proposal would require that products containing this ingredient either be reformulated or withdrawn from the market. Recently, the FDA became aware of data indicating that phenolphthalein is a potential carcinogen in humans. Under the direction of the National Institute of Environmental Health Science (NIEHS) through the National Toxicology Program (NTP), phenolphthalein was studied for its carcinogenic potential in rats and mice. The National Cancer Institute (NCI) nominated phenolphthalein for study because of its widespread chronic use in OTC laxative drug products and the lack of adequate testing for carcinogenicity in experimental animals. The preliminary findings were reported in a 1995 NTP draft technical report (NTP TR 465, NIH publication No. 95-3390), which indicated that phenolphthalein demonstrated evidence of carcinogenic activity in rats and mice. The final version of this report was published in November 1996 (NTP TR 465, NIH publication No. 97-3390).
     In these studies, male and female F344/N rats and B6C3F1 mice were exposed to phenolphthalein (98 percent to 99 percent pure) in feed for 14 days, 13 weeks, or 2 years. Genetic toxicology studies in Salmonella typhimurium (Ames test), cultured Chinese hamster ovary (CHO) cells, and mouse peripheral blood cells were also conducted. Phenolphthalein was not mutagenic in the Ames test and was inactive in the CHO cell sister chromatid exchange assay. It was, however, clastogenic in a CHO cell chromosomal aberration test in the presence of metabolic activation and in the mouse micronucleus assay.
     In the 2-year carcinogenicity studies, groups of 50 male and female rats were given 0, 12,000. 25,000, or 50,000 parts per million (ppm) phenolphthalein in feed for 2 years (equivalent to average daily doses of approximately 500, 1,000, or 2,000 milligrams (mg) phenolphthalein/kilograms (kg) body weight to males and 500, 1,000, or 2,500 mg/kg to females). Groups of 50 male and female mice were given 0, 3,000, 6,000, or 12,000 ppm phenolphthalein in feed for 2 years (equivalent to average daily doses of approximately 300, 600, 1,200 mg phenolphthalein/kg body weight to males and 400, 800, 1,500 mg/kg to females).
     From these 2-year feeding studies, NTP concluded that there was clear evidence of carcinogenic activity of phenolphthalein in male rats based on the markedly increased incidences of benign pheochromocytoma of adenomas and carcinomas. There was some evidence of carcinogenic activity of phenolphthalein in female rats based on the increased incidences of benign or benign and malignant pheochromocytoma of the adrenal medulla. There was clear evidence of the carcinogenic activity of phenolphthalein in male mice based on the increased incidences of histiocytic sarcoma and malignant lymphoma of thymic origin. There was clear evidence of carcinogenic activity of phenolphthalein in female mice based on the increased incidences of histiocytic sarcoma, malignant lymphoma of all types, lymphoma of thymic origin, and benign sex-cord stromal tumors of the ovary. Thus, the 1995 NTP draft technical report on the carcinogenicity studies of phenolphthalein concluded that phenolphthalein has carcinogenic activity in rodents.
     On April 2, 1996 the 1995 NTP draft technical report were discussed at an FDA Center for Drug Evaluation and Research (CDER) Carcinogenicity Assessment Committee (CAC) meeting. A majority of the CAC members agreed that the carcinogenicity studies as conducted provided a valid assessment of carcinogenic potential of phenolphthalein and that the studies addressing genotoxic potential has comparative metabolism and exposure provides information of potential relevance to human risk. The CAC indicated that phenolphthalein is a likely genotoxin for the rodent, but adequate data were not available to make a clear assessment for humans. The CAC concluded that further evaluation of the safety of phenolphthalein should be done following a completion of NTP's pending studies of phenolphthalein including in the p53 transgenic mouse.
     NTP recently completed additional studies on phenolphthalein and prepared draft manuscripts of the findings for publication. The new information and the previous findings were the subject of an April 30, 1997, CAC meeting (the April 30, 1997, meeting). The studies involved five areas: Human epidemiology, in vivo rodents metabolism and distribution, in vitro free radical metabolism, in vitro cell transformation and mutagenicity in Syrian hamster embryo (SHE) cells, and tumorigenicity and micronucleus studies in p53 deficient mice.
     The CAC members voted that the p53 heterozygous mice studies demonstrate that phenolphthalein may be carcinogenic through a genotoxic mechanism. There was a clear dose-dependent increase in the incidence of thymic lymphoma in the p53 assay, confirming one of the primary tumors of concern to the CAC based on its original evaluation of the 2-year assay data. These tumors occurred at doses that showed no other signs of toxicity.
     The CAC believed that several of the assays and data support a genotoxic mechanism. Phenolphthalein was positive in chromosome aberration tests and showed chromosomal abnormality and hypoxanthine phosphoribosyltransferase (hprt) mutations in the SHE cell assay. Nontoxic doses caused cell transformation, mutations, and chromosome aberration. Phenolphthalein was also positive in the peripheral blood micronucleus assay in p53 mice. The micronucleus assay showed that even at the low doses (about 15 times the human exposure), the micronuclei response occurred with increased duration of treatment. It might be expected with a free radical generator, such as phenolphthalein, and based on the observations in mice, that it will take time, at lower concentrations, for lesions to occur and be detected. Thus, it appears that this genotoxic event may be observed with short term phenolphthalein use. The p53 protein accumulation in the nucleus of thymic lymphoma cells of the original bioassay, coupled with the deletion of the wild type p53 allele in the thymic lymphomas of p53 mice, are indicative of interaction with the p53 gene as a target site. In vivo, repeated exposure resulted in micronuclei in both the original bioassay and in p53 mice studies. The exposures used to demonstrate these in vivo and in vitro genotoxic effects were in the range of those that could occur with human laxative use.
     Based on the totality of the evidence that has been evaluated thus far, the FDA considered the use of phenolphthalein a potential risk to humans. These findings of rodent carcinogenicity and genotoxicity in several test systems indicate that chronic use could lead to damage to the human genome (including p53, which is known to be a tumor suppressor gene) and could increase the risk of malignancy. Some human cancers are associated with alterations in the p53 gene. Such genetic damage and increased risk could occur at phenolphthalein doses that are likely to be used by humans.
     Some US producers of phenolphthalein containing laxative products have voluntarily withdrawn their products from the market.

References:

1. Department of Health and Human Services, Food and Drug Administration. Laxative Drug Products for Over-the-Counter Human Use; Proposed Amendment to the Tentative Final Monograph. Federal Register, September 2, 1997; 62(169):46223-46227.
2. NTP Technical Report on the Toxicology and Carcinogenesis Studies of Phenolphthalein in F344/N Rats and B6C3F1 Mice (Feed Studies), NTP TR 465, NIH Publication No. 97-3390, Comment No. RPT8, Docket No. 78N-036L, Dockets Management Branch.

From Saudi Pharmaceutical Journal, Vol 6, No. 1, January 1998