Quantitative determination of mithramycin in human plasma by a novel, sensitive ultra-HPLC–MS/MS method for clinical pharmacokinetic application

• A UPLC–MS/MS method for quantitation of mithramycin in human plasma was developed.
• Solid-phase extraction using mixed-mode sorbent utilized and negative electrospray ionization.
• Lower limit of quantitation was 500 pg/mL.
• This method was applied to samples from patients in two clinical trials at the NCI.
• New data for plasma protein binding of mithramycin are reported.

Mithramycin is a neoplastic antibiotic synthesized by various Streptomyces bacteria. It is under investigation as a chemotherapeutic treatment for a wide variety of cancers. Ongoing and forthcoming clinical trials will require pharmacokinetic analysis of mithramycin in humans, both to see if target concentrations are achieved and to optimize dosing and correlate outcomes (response/toxicity) with pharmacokinetics. Two published methods for mithramycin quantitation exist, but both are immunoassays that lack current bioanalytical standards of selectivity and sensitivity. To provide an upgraded and more widely applicable assay, a UPLC–MS/MS method for quantitation of mithramycin in human plasma was developed. Solid-phase extraction allowed for excellent recoveries (>90%) necessary for high throughput analyses on sensitive instrumentation. However, a ∼55% reduction in analyte signal was observed as a result of plasma matrix effects. Mithramycin and the internal standard chromomycin were separated on a Waters Acquity BEH C18 column (2.1 × 50 mm, 1.7 μm) and detected using electrospray ionization operated in the negative mode at mass transitions m/z 1083.5 → 268.9 and 1181.5 → 269.0, respectively, on an AB Sciex QTrap 5500. The assay range was 0.5–500 ng/mL and proved to be linear (r2 > 0.996), accurate (≤10% deviation), and precise (CV < 15%). Mithramycin was stable in plasma at room temperature for 24 h, as well as through three freeze–thaw cycles. This method was subsequently used to quantitate mithramycin plasma concentrations from patients enrolled on two clinical trials at the NCI.