Cancer Therapy: Preclinical Tramadol and Flurbiprofen Depress the Cytotoxicity of Cisplatin via Their Effects on Gap Junctions

Purpose: Cancer patients are often concurrently treated with analgesics and antineoplastic drugs, yet the influence of analgesic agents on therapeutic activity of antineoplastic drugs is largely unexplored. This study investigates the effects of three commonly used analgesics, which produce analgesia by different mechanisms, on cytotoxicity induced by cisplatin, a widely used antitumor agent, and the relation between those effects and modulation of gap junction function by the analgesics. Experimental Design: The role of gap junctions in the modulation of cisplatin toxicity is explored by manipulation of connexin expression, and gap junction presence and function, using clinically relevant concentrations of the analgesics and cisplatin. Results: Short-term exposure of transformed cells to cisplatin reduced the clonogenic survival in low-density cultures (without gap junction formation) and in high density (with gap junction formation), but the toxic effect was greater at high density. In the absence of connexin expression or with block of connexin channels, cell density had no effect on cisplatin toxicity. Tramadol and flurbiprofen, but not morphine, significantly reduced cisplatin cytotoxicity, but this effect required functional gap junctions between the cells. Tramadol and flurbiprofen inhibited dye-coupling through gap junctions, but morphine did not. Conclusions: The results suggest that the density dependence of cisplatin toxicity is mediated by gap junctions. They further indicate that tramadol and flurbiprofen depress cisplatin cytotoxicity through inhibition of gap junction activity, and more generally, that agents that depress junctional communication can counteract the effects of antitumor agents. (Clin Cancer Res 2009;15(18):5803–10) Relief of pain is an integral component of the care of cancer patients. The cancer as well as antineoplastic therapies can cause substantial pain (1, 2). For this reason, cancer patients are often treated concurrently with analgesics and antineoplastic drugs. However, the influence of analgesic agents on the antitumor activity of antineoplastic drugs, and the mechanism of any such effects, are largely unknown. Cisplatin is one of the most widely used cancer chemotherapy agents. It is particularly effective in the treatment of testicular cancers and is also effective against carcinomas of many other tissues, including those of lung, head and neck, cervix, and ovary. Historically, cisplatin is thought to exert its primary cytotoxic effects via formation of platinum-DNA adducts, leading to G2 cell cycle arrest and apoptosis (3, 4). Recently, other mechanisms have come to light, including rapid formation of DNAprotein cross-links (5) and generation of reactive oxygen species and oxidative stress with a variety of apoptotic, mitochondrial, endoplasmic reticulum, and cell cycle consequences (6). Gap junctions directly connect the cytoplasms of neighboring cells, thereby mediating direct intercellular movement of cytoplasmic signaling molecules. Almost every cellular and tissuelevel process is affected by this communication pathway, including differentiation, migration, and apoptosis (7, 8). Many studies have shown that gap junctions play important roles in cancer biology and drug resistance (9, 10). It has been reported that cisplatin toxicity is enhanced by the presence of functional gap junctions between the target cells (11). Also, gap junctions were shown to enhance induction of apoptosis induced by etoposide (an agent used with cisplatin in the standard combination bleomycin-etoposidecisplatin Authors' Affiliations: Department of Pharmacology, Zhongshan School of Medicine, Sun Yat-Sen University; Departments of Anaesthesia and General Surgery, The Second Affiliated Hospital, Sun Yat-sen University, Guangzhou, People's Republic of China; and Department of Pharmacology and Physiology, New Jersey Medical School, Newark, New Jersey Received 3/31/09; revised 5/19/09; accepted 5/20/09; published OnlineFirst