Remarkable enhancement of cytotoxicity of onconase and cepharanthine when used in combination on various tumor cell lines

Abstract

Onconase (Onc), a ribonuclease from oocytes or early embryos of Northern Leopard frog (Rana pipiens), is cytostatic and cytotoxic to a variety of tumor lines in vitro, inhibits growth of tumors in animal in vivo models and is currently in Phase IIIb clinical trials for malignant mesothelioma where it displays antitumor activity with minor overall toxicity to the patient. One of the characteristic features of Onc is a synergism with a variety of other antitumor modalities. Cepharanthine (Cep), a biscoclaurine alkaloid from Stephania cepharantha Hayata, is widely used in Japan to treat variety of ailments. It also shows low toxicity to patients. The aim of the present study was to assess the interaction of these two drugs on different tumor cell lines. When human promyelocytic leukemia HL-60, histiomonocytic lymphoma U937, multiple myeloma RPMI-8228, prostate carcinoma DU 145 and prostate adenocarcinoma LNCaP cells were exposed to relatively low concentrations of Onc or Cep their growth rates were somewhat suppressed but the cells were still able to proliferate. Cell growth, however, was totally abolished in each of these cell lines when treated with Onc and Cep combined. The frequency of apoptosis was also many-fold higher in cultures treated with a combination of Onc and Cep than in respective cultures treated with Onc or Cep alone. The mechanism of the observed synergism is unclear but it may be associated with the Onc activity in targeting microRNAs and/or NFkappaB and Cep activity also targeting NFkappaB. The data suggest that the combination of these two drugs, that individually express a low toxic profile, may have strong antitumor potential.

Figure 1

Effect of Cep (Ce), or Onc either alone or in combination (Ce + Onc) on growth of HL-60, U937, RPMI-8228, DU 145 and LNCaP cells. The drugs were administered into cultures at time 0, Cep at concentration either 1 or 5 μg/ml (Ce; Ce 5) and Onc at concentration either 5 or 10 μg/ml (Onc 5, Onc 10), and frequency of live (trypan blue excluding) cells was estimated after 1, 2, 3 and 4 days of culturing. Note no cell growth in cultures treated concurrently with Cep and Onc.

Figure 2

Frequency of apoptosis in cultures of HL-60, U937, RPMI-8228, DU 145 and LNCaP cells treated with Cep (Ce) or Onc either alone or in combination (Ce + Onc). The drugs were added into cultures at time zero, Cep at concentration 5 μg/ml (Ce; Ce 5) and Onc at concentration either 5 or 10 μg/ml (Onc 5, Onc 10), as shown, and the frequency of apoptotic (TUNEL positive; percent shown in each panel) cells in cultures of HL-60, U937 and RPMI-8228 was estimated 24 h, and in cultures of DU 145 and LNCaP 72 h-after drug administration.