Quantification of Polyethylene Glycol Esters of Methotrexate and Determination of Their Partition Coefficients by Validated HPLC Methods

Conjugation of methotrexate (MTX) (MW 454) with different molecular weights of polyethylene glycol (PEG) including methoxy-peg (mpeg) 750 D and 5000 D and diolpeg 35000 D led to compounds that are physicochemically highly different from the parent compound, MTX. In this study, an HPLC system consisting of C8 column and UV detector (λ=342 nm), using a mixture of 30:70 v/v phosphate-citrate buffer (pH 4): methanol mobile phase, was validated for quantification of the esters. Three other HPLC methods using three mixture of buffer phosphate-citrate (pH 6): methanol at 30:70, 40:60 and 50:50 v/v, respectively, was set for estimation of partition coefficients the esters. Eight reference standard materials were selected from literature covering the retention times lower and higher than esters. An identical log P (4.3) was obtained for all three esters, despite their different molecular weights (i.e. 1200, 5500 and 35500 D theoretically). In addition the log P obtained differs from that of the parent drug (-1.4). This high difference comes probably from different ability of drug and esters in ionization of carboxylic acid groups.