Dl. Coleman et al., A SENSITIVE AND SPECIFIC RADIOIMMUNOASSAY FOR LY309887, A POTENT INHIBITOR OF GLYCINAMIDE RIBONUCLEOTIDE FORMYLTRANSFERASE, Clinical cancer research, 4(1), 1998, pp. 157-163
LY309887, a reduced analogue of folic acid, is a potent inhibitor of g
lycinamide ribonucleotide formyltransferase and possesses a broad spec
trum of antitumor activity. During preclinical studies using supplemen
tation with oral folic acid, this second-generation inhibitor displaye
d both the desired safety profile and the pharmacology to warrant clin
ical investigation. A sensitive analytical method was needed to assess
the pharmacokinetics of LY309887 due to the low doses planned for Pha
se I studies and the potential for low concentrations in plasma long a
fter i.v. administration. We therefore undertook the development of a
competitive RIA. A highly specific antiserum was raised in rabbits fol
lowing immunization with LY309887 coupled to BSA. A RIA tracer was pre
pared by radioiodination of compound 389753, the adduct of LY309887 wi
th p-tyramine. We developed a competitive-binding RIA procedure and us
ed superparamagnetic particles coated with goat antirabbit IgG as a me
thod for separating the bound and free forms of LY309887. The RTA is s
ensitive (0.5 ng/ml in serum and 25 ng/ml in urine), specific (negligi
ble interference from endogenous folates), and reproducible (interassa
y coefficients of variation ranging from 8.1 to 15.4% and 7.6 to 8.3%
for serum and urine controls, respectively), We used the RIA to assess
the i.v. pharmacokinetics of LY309887 in both patients with metastati
c cancer and dogs. The sensitivity of the RIA permitted the demonstrat
ion that serum concentrations of LY309887 decline in a multiexponentia
l manner with a prolonged terminal elimination phase, We conclude that
the RIA is a valid method for quantifying LY309887 in biological flui
ds.