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-pyrrolo[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-H₄, [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-H₄ 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 ≥ 0.999. The intrarun absolute %RE was ≤19.6, 14.2, and 14.0% at 0.056, 2.83, and 56.3 ng/ml, respectively. The corresponding %RSD was ≤14.9%, 9.01, and 4.59%. The interday %RSD was ≤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 ≥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 μg/m², the starting dose of an NCI-sponsored dose escalation study.