Double high-dose chemotherapy with stem cell rescue (HD-SCR) in patients with breast cancer - effect of sequence

Introduction: A preliminary analysis of our double high-dose chemotherapy w ith stem cell rescue (HD-SCR) clinical trial for breast cancer, and preclin ical cross-resistant studies, suggested that melphalan (M) adversely affect ed response to subsequent chemotherapy, i.e., that the sequence of alkylati ng agents (AAs) might affect response. We, therefore, constructed and exami ned preclinical models to determine whether prior exposure to M, in fact, a dversely affected response to other therapy. Purpose: The purpose of the st udy was to determine whether the sequence of AAs, specifically the prior us e of M, adversely affected response to subsequent treatment. Methods: The m ethods employed were the following: (1) Human tumor cell lines rendered res istant by in vitro sequential exposure to five different AAs were developed . The resistant cell lines were examined for cross-resistance to alkylating and other agents. (2) In vivo studies in the p388 mouse leukemia for resis tance and cross-resistance among the AAs. (3) In vivo studies of the effect of sequence of AAs on response in mice bearing EMT6 breast cancer. (4) The double transplant model was developed in the mouse and the sequence of hig h-dose AAs was studied. (5) Biochemical and reverse transcriptase-polymeras e chain reaction (RT-PCR) studies of the various resistant tumor cell lines . Results: (1) The in vitro human tumor cells resistant to M were cross-res istant in 57% of tests to other AAs, In contrast, resistance for other AAs crossed to other agents in only 10 to 20% of tests. (2) The in vivo studies of p388 indicated that resistance to M commonly crossed to other AAs and m any non-AAs, (3) The results for the mouse breast cancer (EMT6) studies of the sequence of AAs again indicated that M employed first markedly reduced responsiveness to subsequent treatment, particularly with AAs. (4) The doub le transplant model: again, M first markedly reduced response to other agen ts. (5) The in vitro resistant human tumor cell lines. particularly the bre ast cancer cell line MCF7, were found to contain high concentrations of glu tathione S1 transferase gamma, which is consistent with that mechanism bein g responsible for resistance. Conclusion: The sequence of alkylating agent treatment may substantially influence response. Melphalan, particularly, pr oduces resistance that commonly crosses to the other AAs. Mechanistic studi es indicate significant changes in glutathione S1 transferase, a known mech anism for broadly based resistance to AAs.