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The breast cancer resistance protein (BCRPABCG2) is a member of the ATP-binding cassette family of drug transporters and confers resistance to various anticancer drugs. We show here that mice lacking Bcrp1Abcg2 become extremely sensitive to the dietary chlorophyll-breakdown product pheophorbide a, resulting in severe, sometimes lethal phototoxic lesions on light-exposed skin. Pheophorbide a occurs in various plant-derived foods and food supplements. Bcrp1 transports pheophorbide a and is highly efficient in limiting its uptake from ingested food. Bcrp1(-/-) mice also displayed a previously unknown type of protoporphyria. Erythrocyte levels of the heme precursor and phototoxin protoporphyrin IX, which is structurally related to pheophorbide a, were increased 10-fold. Transplantation with wild-type bone marrow cured the protoporphyria and reduced the phototoxin sensitivity of Bcrp1(-/-) mice. These results indicate that humans or animals with low or absent BCRP activity may be at increased risk for developing protoporphyria and diet-dependent phototoxicity and provide a striking illustration of the importance of drug transporters in protection from toxicity of normal food constituents.

The mdr1-type P-glycoproteins (P-gps) confer multidrug resistance to cancer cells by active extrusion of a wide range of drugs from the cell. To study their physiological roles, we have generated mice genetically deficient in the mdr1b gene [ mdr1b (−/−) mice] and in both the mdr1a and mdr1b genes [ mdr1a/1b (−/−) mice]. In spite of the host of functions speculatively attributed to the mdr1-type P-gps, we found no physiological abnormalities in either strain. Viability, fertility, and a range of histological, hematological, serum–chemical, and immunological parameters were not abnormal in mdr1a/1b (−/−) mice. The high level of mdr1b P-gp normally present in the pregnant uterus did not protect fetuses from a drug (digoxin) in the bloodstream of the mother, although the protein did reduce drug accumulation in the adrenal gland and ovaries. Pharmacologically, mdr1a/1b (−/−) mice behaved similarly to the previously analyzed mdr1a (−/−) mice, displaying, for instance, increased brain penetration and reduced elimination of digoxin. However, both mdr1a and mdr1b P-gps contributed to the extrusion of rhodamine from hematopoietic progenitor cells, suggesting a potential role for the endogenous mdr1-type P-gps in protection of bone marrow against cytotoxic anticancer drugs. This, and the normal viability of mdr1a/1b (−/−) mice, has implications for the use of P-gp-blocking agents in cancer and other chemotherapy. mdr1a/1b (−/−) mice should provide a useful model system to further test the pharmacological roles of the drug-transporting P-gps and to analyze the specificity and effectivity of P-gp-blocking drugs.

Cytochrome P450 3A (CYP3A) enzymes constitute an important detoxification system that contributes to primary metabolism of more than half of all prescribed medications. To investigate the physiological and pharmacological roles of CYP3A, we generated Cyp3a-knockout (Cyp3a -/-) mice lacking all functional Cyp3a genes. Cyp3a -/-mice were viable, fertile, and without marked physiological abnormalities. However, these mice exhibited severely impaired detoxification capacity when exposed to the chemotherapeutic agent docetaxel, displaying higher exposure levels in response to both oral and intravenous administration. These mice also demonstrated increased sensitivity to docetaxel toxicity, suggesting a primary role for Cyp3a in xenobiotic detoxification. To determine the relative importance of intestinal versus hepatic Cyp3a in first-pass metabolism, we generated transgenic Cyp3a -/-mice expressing human CYP3A4 in either the intestine or the liver. Expression of CYP3A4 in the intestine dramatically decreased absorption of docetaxel into the bloodstream, while hepatic expression aided systemic docetaxel clearance. These results suggest that CYP3A expression determines impairment of drug absorption and efficient systemic clearance in a tissue-specific manner. The genetic models used in this study provide powerful tools to further study CYP3A-mediated xenobiotic metabolism, as well as interactions between CYP3A and other detoxification systems.