The activity of deoxyspergualin in multidrug-resistant cells

Deoxyspergualin, a synthetic analogue of the immunosuppressive anti-tumour antibiotic spergualin, has been shown to possess potent in vitro and in vivo anti-tumour activity and is currently in the National Cancer Institute (NCI) decision network. Deoxyspergualin shows similarities in properties and mechanisms of action to the natural-product immunosuppressive agents cyclosporin A and FK506, each of which can act as a modifier of multidrug resistance. We therefore decided to examine the comparative activity of deoxyspergualin in parent and multidrug-resistant cells. Deoxyspergualin contains the polyamine spermidine within its structure. Bovine serum copper amine oxidase catalyses the oxidative deamination of spermidine to produce an aminoaldehyde, ammonia and hydrogen peroxide. These aminoaldehydes are believed to be responsible for the toxicity of polyamines in vitro in the presence of bovine serum. For this reason, all experiments were carried out in medium containing bovine serum and in medium containing horse serum (which is low in copper amine oxidase content). We used the tetrazolium (MTT) colorimetric assay to determine drug sensitivity and tritiated daunorubicin accumulation together with inhibition of azidopine binding to study specific mechanisms of resistance modulation. The murine cell lines EMT6/P and EMT6/AR1.0 and the human cell lines H69/P and H69/LX4 were, respectively, 32-, 32-, 372- and 483-fold more sensitive to spermidine and 175-, 133-, 321- and 444-fold more sensitive to spermine in the presence of calf serum than in the presence of horse serum. However, these large differential effects were not seen for deoxyspergualin. It appears that in the presence of horse serum, deoxyspergualin may exert its effect by a mechanism other than polyamine oxidation. Deoxyspergualin did not enhance the accumulation of [3H]-daunorubicin in EMT6/AR1.0 cells. Furthermore, deoxyspergualin (1-20 microM) did not restore the sensitivity of EMT6/AR1.0 or H69/LX4 cells to that of the parent lines. P-glycoprotein (Pgp) in membranes prepared from H69/LX4 cells was photo-affinity-labeled with [3H]-azidopine. Deoxyspergualin did not inhibit this labeling. Although deoxyspergualin appears to exert its immunosuppressive effect via a mechanism similar to that of cyclosporin A and FK506, it does not share their ability to modify Pgp-mediated multidrug resistance. However, its lack of cross-resistance and potent in vivo anti-tumour activity make deoxyspergualin a promising candidate for development as an anti-cancer agent.