The Activity of Combined Topoisomerase Inhibitors Is Augmented by Octreotide in the Treatment of Lymphoid Leukemias.

Clinical protocols combining a topoisomerase I (topo I) and a topoisomerase IIalpha (topo II inhibitor) have shown major responses against several tumours including acute leukemia and myelodysplastic syndromes. However, combinations with topoisomerase targeting drugs should be considered with caution because antagonistic effects have been observed when administering camptothecin or topotecan (topo I inhibitors) with doxorubicin (topo II inhibitor). Octreotide (OCT) is an eight amino-acid peptide, which attains its biological effects on target cells by binding preferentially to sst2 and, to a lesser extent, to sst3 and sst5 somatostatin receptors (SS-Rs). The presence of SS-Rs in human lymphoid leukemia cell lines, in malignant lymphomas and in lymphoproliferative diseases is clearly detected. We studied the in vitro effect of combinations of OCT with doxorubicin (DOX) and topotecan (TP) on cell growth and viability in four human lymphoblastic leukemia cell lines (CCRF-CEM, RPMI-8226, JURKAT, MOLT-4) and on the Topo I, IIalpha and sst2 expression in JURKAT, MOLT-4 leukemia cell lines, as well as the in vivo effect on rodent P388 lymphocytic and L1210 lymphoid leukemias. In vitro growth inhibition and cytotoxicity were evaluated with the MTT colormetric metabolic assay. Topo I, IIalpha and sst2 mRNAs were detected with RT-PCR and the quantification of the electrophoresed specific PCR products was accomplished with Molecular Imager FX. In vivo antitumour activity was estimated by the % survival ratio of treated to untreated (control) mice and the ratio of 70-day tumour free survivors (cures). The in vitro growth inhibition and cytotoxicity that were induced by the DOX and TP combinations were neither synergistic nor additive and were similar to the activity of DOX alone. However, with the addition of OCT to the DOX and TP combinations a significant (p<0.001) synergistic effect was resulted in all tested cell lines. Treatment of cell lines with DOX produced almost a total consumption of Topo I and IIalpha mRNAs and with TP induced increase of Topo IIalpha mRNA levels (0.5–1.5 folds). OCT is clearly upregulates the Topo IIalpha expression (>2.5 folds) and restores Topo I mRNA production in cells treated with DOX. The sst2 mRNA levels were not affected in any case. In vivo antitumour activity of DOX and TP combinations was neither synergistic nor additive and it was similar to the activity of DOX alone. The addition of OCT to DOX and TP combinations produced an important synergistic antitumour effect increasing significantly survival time and cures (p<0.01) in both P388 and L1210 leukemias. Our data indicate that the antagonistic effects of Topo I and Topo II inhibitors may be due to effects on the regulation of topoisomerases expression. OCT significantly enhances the antileukemic activity of combinations with such important anticancer drugs, upregulating Topo I and IIalpha expression.