Determination of chemically reduced pyrrolobenzodiazepine SJG-136 in human plasma by HPLC-MS/MS: application to an anticancer phase I dose escalation study.

SJG-136 1,1'-[[(propane-1,3-diyl)dioxy]bis[(11aS)-7-methoxy-2-methylidene-1,2,3,11a-tetrahydro-5H-pyr- rolo[2,1-c][1,4]benzodiazepin-5-one]] (NSC 694501), is a bifunctional pyrrolobenzodiazepine (PBD) dimer that forms selective, irreversible, interstrand DNA cross-links via exocyclic N2 atoms of two guanine bases, with a preference for 5'PuGATCPy binding sites. SJG-136 is highly cytotoxic in human tumor cells in vitro and in human tumor xenograft models in vivo at subnanomolar concentrations and is currently in anticancer phase I clinical trials in the United Kingdom and United States. To support correlative pharmacokinetics studies, a highly sensitive HPLC-MS/MS assay was developed and validated for the reliable quantitation of SJG-136 in human plasma, using the structurally similar PBD dimer DSB-120 as an internal standard. Chemical reduction of SJG-136 to its corresponding amine (SJG-136-H(4), [M + H](+)m/z 561) improved HPLC peak resolution and sensitivity by minimizing complications that arose from the reactivity of the labile imine moieties. Plasma samples were processed by protein precipitation and centrifugal membrane dialysis; components were separated by HPLC using an Agilent Rapid Resolution HT 1.8 mm (2.1 mm x 50 mm) analytical column. The total analysis time from injection to injection was 11 min. Electrospray MS/MS detection of SJG-136-H(4) was based on the selected reaction monitoring (SRM) transition [M + H](+)m/z 561 --> 301. The analytical response ratio was linearly proportional to the plasma concentration of SJG-136 over the nominal concentration range of 25 pg/ml to 250 ng/ml, with a coefficient of determination of r > or = 0.999. The intrarun absolute %RE was < or =19.6, 14.2, and 14.0% at 0.056, 2.83, and 56.3 ng/ml, respectively. The corresponding %RSD was < or =14.9%, 9.01, and 4.59%. The interday %RSD was < or =2.72, 3.46, and 5.20%. The lower and upper limits of quantitation were 0.056 and 56 ng/ml, respectively; recovery of SJG-136 from plasma was > or = 62% across the validated concentration range. The sensitivity of the validated assay was sufficient to detect SJG-136 in human subjects for up to 6 h after intravenous administration of 6 microg/m(2), the starting dose of an NCI-sponsored dose escalation study.

Stoichiometric preference in copper-promoted oxidative DNA damage by ochratoxin A.

The ability of the fungal carcinogen, ochratoxin A (OTA, 1), to facilitate copper-promoted oxidative DNA damage has been assessed using supercoiled plasmid DNA (Form I)-agarose gel electrophoresis and gas chromatography-mass spectrometry with selected-ion monitoring (GC-MS-SIM). OTA is shown to promote oxidative cleavage of Form I DNA with optimal cleavage efficiency occurring under excess Cu(II) conditions. As the concentration of OTA was increased and present in excess of Cu(II) the cleavage was less effective. Parallel findings were found for the ability of the OTA-Cu mixture to facilitate oxidative base damage. Yields (lesions per 10(6) DNA bases) of modified bases upon exposure of calf-thymus DNA (CT-DNA) to OTA-H(2)O(2)-Cu(II) were diminished when the OTA:Cu ratio was increased to 5:1. Electrochemical studies carried out in methanol implicate a ligand-centered 2e oxidation of OTA in the presence of excess Cu(II), while product analyses utilizing electrospray mass spectrometry support the intermediacy of the quinone, OTQ (3), in Cu-promoted oxidation of OTA. The implications of these findings with regard to the mutagenicity of OTA are discussed.