ORIGINAL ARTICLE Frank Bohnenstengel á Godehard Friedel Christoph A. Ritter á Monika McClellan Peter Fritz á Michel Eichelbaum Albert Linder á Heikki Toomes Rainer Dierkesmann á Heyo K. Kroemer Variability of cyclophosphamide uptake into human bronchial carcinoma: consequences for local bioactivation Received: 4 March 1999 / Accepted: 8 June 1999 Abstract Purpose: The alkylating cytostatic prodrug cyclophosphamide is bioactivated by the human cyto- chrome P450 enzyme system. Since these enzymes are not only expressed in human liver, but also in extra- hepatic tissue, local bioactivation of this drug may play an important role in its antineoplastic eects, e.g., che- motherapy of lung tumors. This would require uptake of signi®cant amounts of cyclophosphamide into tumor tissue, which has not yet been demonstrated. Methods: We used a recently developed, ex vivo isolated, ventilated and perfused human lung model to study cyclophos- phamide uptake into bronchial carcinoma and healthy lung tissue. Following a standard lobectomy, lung sam- ples containing the tumor were perfused with buer containing 2 mM cyclophosphamide for 2 h. Cyclo- phosphamide concentrations in perfusate and healthy peripheral tissue were measured during the perfusion and in tumors at the end of perfusion. Results: In all tissue samples, cyclophosphamide uptake was relatively poor, indicated by a tissue to perfusate ratio of 0.021. More- over, in tumor samples, cyclophosphamide concentra- tions were signi®cantly lower (P < 0.05) than in healthy lung tissue and showed pronounced interindividual variability. Median concentrations were 36.8 lg/g (26.9± 44.2 lg/g) in healthy tissue and 5.1 lg/g (0.0±26.8 lg/g) in tumor samples. Tumor cyclophosphamide concen- trations varied between 0 and 75% of those reached in healthy tissue. Conclusions: Our results indicate that CP tumor concentrations are modulated by factors dierent from dose and that expression of bioactivating enzymes in human lung or transfection of genes encoding these enzymes into tumor cells does not necessarily lead to local bioactivation of cyclophosphamide. Key words Cyclophosphamide á Human lung tissue á Pharmacokinetics Introduction The lack of tumor selectivity is one of the major limi- tations of cancer chemotherapy. To improve tumor re- sponse, doses of cytostatics have been escalated in the last years, resulting in multiple dose-limiting adverse eects [26]. Among various strategies to improve drug targeting, the use of non-cytotoxic prodrugs, which are mainly activated at the tumor site, appears to be promising. One of the most common prodrugs is the ox- azaphosphorine cytostatic cyclophosphamide, which is widely used in chemotherapy of various malignancies. It is well established that the parent compound is bioacti- vated, and also deactivated, by the cytochrome P450 (CYP) enzyme system in human liver. The ®rst step, a 4-hydroxylation (ring oxidation), which is catalyzed by CYP 2B6, 2C, and to a lesser extent by CYP 3A [6], leads to the formation of 4-hydroxycyclophosphamide. This compound is in equilibrium with its ring-opened tautomer aldophosphamide, which spontaneously de- composes to yield phosphoramide mustard, the ultimate alkylating agent, and the urotoxic compound acrolein. Besides bioactivation, there are two other pathways that lead via oxidation of 4-hydroxycyclophosphamide or aldophosphamide to the inactive metabolites 4-ketocy- Cancer Chemother Pharmacol (2000) 45: 63±68 Ó Springer-Verlag 2000 F. Bohnenstengel á M. Eichelbaum Dr. Margarete Fischer-Bosch-Institut fuÈr Klinische Pharmakologie, Auerbachstrasse 112, D-70376 Stuttgart, Germany M. McClellan á P. Fritz Pathologisches Institut am Robert Bosch-Krankenhaus, Auerbachstrasse 110, D-70376 Stuttgart, Germany G. Friedel á H. Toomes á R. Dierkesmann Klinik SchillerhoÈhe der LVA WuÈrttemberg, D-70839 Gerlingen, Germany A. Linder Lungenklinik Hemer, Theo-Funccius-Strasse 1, D-58675 Hemer, Germany C.A. Ritter á H.K. Kroemer (&) Institut fuÈr Pharmakologie, Friedrich-Loeer-Strasse 23 D, D-17487 Greifswald, Germany Tel.: +49-3834-865630; Fax: +49-3834-865631