PI‐75

BACKGROUND Actinomycin‐D (Act‐D) and vincristine (VCR) are used in treating pediatric cancer. Empiric dosing continues due a lack of information on drug disposition. We report a major modification to our published Act‐D and VCR LC/MS/MS assay that has reduced our lower limit of quantification (LLOQ), and decreased run time. METHODS The HPLC system consists of a Waters 2690 LC separation module coupled to an API 4000 MS/MS spectrometer, with a Thermo Hypurity analytical column. Mobile phase consists of water with ammonium formate 5 mM at pH 3.5 (A), and methanol:acetonitrile in a 60:40 (v:v) mix (B). Gradient elution rate is 0.15 μL/min, and run time is 9 minutes. Act‐D elutes at 7.4 minutes, and its internal standard elutes at 6.3 minutes; VCR and its internal standard elute at 2 minutes. Mass spectroscopy is carried out under positive electrospray ionization and multiple reaction monitoring (MRM) mode. Nitrogen is used as a nebulizer gas. Ion recording uses the Q3 ion of Act‐D and its internal standard at m/z 858.3 and 873.2, respectively, and m/z 765.4 and 751.4, for VCR and its standard, respectively. RESULTS The standard curve is linear from 0.1 to 10 ng/mL for Act‐D and VCR, representing a five‐fold lowering in LLOQ from prior assays. CONCLUSIONS We have improved upon our previous LC/MS/MS method, and are capable of quantifying Act‐D and VCR in plasma to 0.1 ng/mL, in half the time previously required. This method will be used in our planned multicenter population PK trial of these agents in children with cancer. Clinical Pharmacology & Therapeutics (2005) 79 , P26–P26; doi: 10.1016/j.clpt.2005.12.096