Biochemical correlates of temozolomide sensitivity in pediatric solid tumor xenograft models

The antitumor activity of the methylating agent temozolomide has been evalu ated against a panel of 17 xenografts derived from pediatric solid tumors. Temozolomide was administered p.o. daily for five consecutive days at a dos e level of 66 mg/kg, Courses of treatment were repeated every 21 days for t hree cycles. Tumor lines were classified as having high, intermediate, or l ow sensitivity, determined by complete responses, partial responses, or sta ble disease, respectively. Overall, temozolomide induced complete responses in five lines and partial responses in three additional tumor lines, givin g objective regressions in 47 % of xenograft lines. Analysis of temozolomid e plasma systemic exposure indicated that this dose level was relevant to e xposure achieved in patients. Tumors were analyzed by immunoblotting for le vels of O-6-methylguanine-DNA methyltransferase (MGMT) and two mismatch rep air proteins, MLH-1 and MSH-2, Tumors classified as having high or intermed iate sensitivity had low or undetectable MGMT and expressed detectable MLH- 1 and MSH-2 proteins. Tumors classified as having low sensitivity had eithe r (a) high MGMT or (b) low or undetectable MGMT but were deficient in MLH-1 . The relationship between p53 and response to temozolomide was also examin ed, In vitro temozolomide did not induce p21(cip1) in p53-competent NB-1643 neuroblastoma cells. Suppression of p53 function in NB1643 clones through stable expression of a trans dominant negative p53 (NB1643p53(TDN)) did not confer temozolomide resistance. Similarly, tumor sensitivity to temozolomi de did not segregate with p53 genotype or p53 functional status. These resu lts indicate that MGMT is the primary mechanism for temozolomide resistance , but in the absence of MGMT, proficient mismatch repair determines sensiti vity to this agent.