IN-VITRO PHOTOINDUCED CYTOTOXICITY AND DNA-BINDING PROPERTIES OF PSORALEN AND COUMARIN CONJUGATES OF NETROPSIN ANALOGS - DNA SEQUENCE-DIRECTED ALKYLATION AND CROSS-LINK FORMATION

The synthesis, DNA binding and in vitro photoinduced cytotoxic propert ies of a number of minor groove and sequence-directed psoralen and cou marin conjugates of pyrrole- and imidazole-containing distamycin analo gues 2-5 are described. Results from an ethidium displacement assay on calf thymus and T4 DNA suggest that like distamycin these agents bind strongly to the minor groove of DNA. The data show that these conjuga tes exhibit a lower AT preference than distamycin and the decrease is significantly greater for the imidazole-containing compounds. All of t he compounds along with 8-methoxypsoralen, 1, were relatively noncytot oxic in the dark with only the imidazole-psoralen compound 3 giving an IC50 value below 100 mu M. Following UV activation, all compounds sho wed an increased potency with photoinduced dose modifications in the h uman chronic myeloid leukemia K562 cells of >333, 12, >1.3, and >2.5 f or compounds 2-5, respectively, under the UV irradiation conditions em ployed. The psoralen-pyrrole analogue 2 was over 300 times more active following UV activation than agent 1,250 times more potent than the c orresponding coumarin conjugate 4, and 15-fold more potent than its im idazole analogue 3. Data from CD dilution (with DMF) studies show that upon irradiation with light at 366 nm, compounds 2-5 bind irreversibl y to DNA. Furthermore, upon irradiation compound 2 produced interstran d cross-linked DNA in quantitative yield, with isolated DNA, at >300- and >3000-fold lower drug concentrations than the imidazole analogue 3 and 8-methoxypsoralen, respectively. As expected coumarin conjugates 4 and 5 did not produce any cross-linked DNA under any conditions. Sin ce the psoralen conjugates are more phototoxic than their coumarin ana logues, these results suggest that DNA interstrand cross-link formatio n may be an important mechanism by which they exert their biological a ctivity in cells. In addition, the enhanced photocytotoxic potency of conjugate 2 over 3 may be related to its larger binding constant, more efficient DNA cross-linking ability, and possibly to its preference f or AT-rich sequences.