Mechanism of melphalan crosslink enhancement by misonidazole pretreatment.

Sensitization of Chinese hamster ovary cells to melphalan (L-PAM) toxicity by prior treatment with misonidazole (MISO, 5 mM, 2 hr, hypoxic conditions, 37 degrees C) is associated with increased levels of DNA crosslinks believed to be the critical lesion for bifunctional alkylating agent toxicity. Enhanced L-PAM crosslinking of DNA could occur by a variety of mechanisms in MISO-pretreated cells including: (1) increased transport or binding of L-PAM, (2) decreased repair of L-PAM monoadducts which would allow more time for their conversion to crosslinks, (3) decreased crosslink repair (unhooking of one arm), or (4) chemical modification of the DNA structure, presumably by bound MISO derivatives, such that crosslink formation is facilitated. Previous studies have eliminated mechanisms (1) and (3). Mechanism (4) was investigated by following MISO-pretreatments of whole cells with L-PAM treatments of the isolated DNA from these cells. This was accomplished by using a modification of the alkaline elution assay for DNA crosslink measurement in which a 1 hr treatment with L-PAM (0-12 micrograms/ml) was inserted between the cell lysis steps and DNA elution procedure. Treatment of bare DNA with L-PAM modeled very well the crosslinking behavior in whole cells although it was somewhat more efficient (more crosslinks at a given L-PAM dose). In the presence of double stranded DNA and absence of repair systems during and after the L-PAM exposure, it was determined that MISO-pretreatments did not increase the crosslinking efficiency of L-PAM (mechanism [4] above). Inhibition of repair of L-PAM monoadducts (mechanism [2] above) still remains as a possible means for crosslink enhancement by MISO-pretreatment.