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Optimizing the Erythromycin Breath Test for Use in Cancer Patients¹

Medical Oncology, Sydney Cancer Centre, New South Wales 2050 [L. P. R., M. B., P. J. B., M. J. M., J. F. B., S. J. C.], and Department of Pharmacology, University of Sydney, New South Wales 2006 [L. P. R., K. S., J. P. S., J. M. H.], Australia

The erythromycin breath test (EBT) is a putative in vivo probe for drug metabolism by cytochrome P450 3A4 (CYP3A4). Because many anticancer drugs are metabolized by this system, we sought to further develop the EBT as a tool for predicting the clearance, in cancer patients, of drugs metabolized by CYP3A4.

Sixteen adult patients with incurable cancer were studied. The EBT was performed on day 1 and breath sampled after the i.v. injection of 4 µCi of ¹⁴C-erythromycin. The breath ¹⁴CO₂ flux (CER_t) was estimated at 11 time points over 2 h. On day 2, the EBT was repeated midway through a 10-min infusion of 100 mg of erythromycin lactobionate, and the plasma pharmacokinetics of erythromycin were determined.

The infusion of 100 mg of erythromycin did not modify the EBT results significantly. The values of the conventional EBT parameter CER_{20 min} obtained on day 1 were comparable for most subjects (0.03–0.06% dose/min), with the exception of an individual receiving the known CYP3A4 inducers dexamethasone and phenytoin who returned a value of 0.14% dose/min. There was no significant correlation between any of the conventional EBT parameters and erythromycin clearance. However, two parameters reflecting early emergence of breath radioactivity (1/T_MAX and CER_{3 min}/CER_MAX) correlated significantly with erythromycin clearance (P = 0.005 and 0.006, respectively).

Novel parameters derived from the EBT are significantly correlated with the clearance of erythromycin even in the presence of confounding factors, such as metastatic liver disease, altered protein binding, and comedication. These parameters may enable dose optimization of cytotoxics metabolized by CYP3A4.

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