The pharmacokinetics of ifosfamide exhibit considerable interindividual variation. It is a prodrug that is extensively metabolised, chiefly by cytochrome P450 isoenzymes such as CYP3A4 and CYP2B6 in the liver, to both active and inactive metabolites; there is some evidence that metabolism is saturated at very high doses. Although licensed product information states that a mean terminal elimination half-life is about 15 hours after a single high-dose intravenous bolus, most studies at lower doses recorded elimination half-lives of 4 to 8 hours. After repeated doses there is a decrease in the elimination half-life, apparently due to autoinduction of metabolism. It is excreted largely in urine, as unchanged drug and metabolites.
Martindale Complete Drug Reference. Pharmaceutical Press (via Medicines Complete), latest modification: 20-Aug-2010
Wagner T. Clin Pharmacokinet 1994; 26: 439–56; Boddy AV, Yule SM. Clin Pharmacokinet 2000; 38: 291–304; Kerbusch T, et al. Clin Pharmacokinet 2001; 40: 41–62.

In patients (n=32) with advanced solid tumours, continuous once daily sunitinib, in escalating doses per cohort, was combined with ifosfamide, 9 g/m2 for 3 days or 6 g/m2 for 5 days, administered every 3 weeks. Neutropenia-related adverse events were dose-limiting toxicities. Sunitinib did not affect ifosfamide PK. Ifosfamide significantly decreased exposure to sunitinib and increased exposure to its metabolite, SU12662. No consistent changes in PD parameters were observed. Ifosfamide produced decreased sunitinib blood levels due to CYP3A induction. As PK interactions cannot explain the relatively low sunitinib doses that can be combined with ifosfamide, synergy in toxicity is likely. Whether this is true for anti-tumour activity needs to be further explored.
Decreased exposure to sunitinib due to concomitant administration of ifosfamide: results of a phase I and pharmacokinetic study on the combination of sunitinib and ifosfamide in patients with advanced solid malignancies. Hamberg P, Steeghs N, Loos WJ et al. Br J Cancer. 2010 Jun 8;102(12):1699-706.

An in vitro study screened 18 widely used anticancer drugs for their ability to activate PXR-mediated CYP3A4 induction. A CYP3A4 reporter gene assay was employed to identify PXR agonists among the eighteen anticancer drugs. Subsequently CYP3A4 mRNA and protein expression following treatment with these PXR agonists was assessed. Finally, the effect of pre-treatment with these agents on the 1'-hydroxylation of midazolam (a specific CYP3A4 probe) was determined. Ifosfamide was able to activate PXR and may therefore have the propensity to cause clinically relevant drug-drug interactions as a result of CYP3A4 induction.
Nuclear receptor mediated induction of cytochrome P450 3A4 by anticancer drugs: a key role for the pregnane X receptor. Harmsen S, Meijerman I, Beijnen JH, Schellens JH. Cancer Chemother Pharmacol. 2009 Jun;64(1):35-43

In vitro studies have demonstrated that conversion to the active metabolite occurs via CYP3A4. Both CYP3A4 and CYP2B6 are involved in detoxification. CYP3A4 metabolism of (S)-ifosfamide may generate a neurotoxic metabolite.
Granvil CP, Madan A, Sharkawi M, et al. Drug Metab Dispos1999; 27: 533-41; Roy P, Tretyakov O, Wright J, et al. Drug Metab Dispos 1999; 27: 1309-18

CYP3A4 activities in human liver microsomes from 2 donors were determined using the oxidation of the dihydropyridine denitronifedipine, a specific CYP3A4 substrate, at a concentration of 50 microM (= KM). Formation of the pyridine metabolite was measured using HPLC. Ifosfamide showed an inhibitory effect on CYP3A4, IC50 3.6/2.5 mmol/l for 2 microsome sample. Comparing IC50 values with plasma concentrations present during antineoplastic therapy, ifosfamide could possibly cause clinical drug interactions by inhibition of CYP3A4.
Screening for inhibitory effects of antineoplastic agents on CYP3A4 in human liver microsomes. Baumhäkel M, Kasel D, Rao-Schymanski RA, Böcker R et al. Int J Clin Pharmacol Ther. 2001 Dec;39(12):517-28

Ketoconazole given orally for 4 days, starting 1 day before intravenous ifosfamide (given as a 24-hour infusion), decreased ifosfamide clearance. However, ifosfamide metabolism to active metabolites was decreased, and urinary excretion of ifosfamide was increased. Ketoconazole may decrease the therapeutic efficacy of ifosfamide.
Kerbusch T, et al. Modulation of the cytochrome P450-mediated metabolism of ifosfamide by ketoconazole and rifampin. Clin Pharmacol Ther 2001; 70: 132–41.