Antitumour synergism between non-toxic dietary combinations of isotretinoin and glucarate.

Dietary calcium glucarate (CGT) increased the activity of non-toxic levels of dietary isotretinoin against pre-established tumors in the chemically-induced rat mammary tumour model. In the range of 1.0-1.5 mmol/kg diet, isotretinoin enhanced tumour growth by 20% over a 4 week course of treatment. Tumour growth inhibition not exceeding 15% was observed only at dosages as high as 2.0 mmol/kg, i.e. in the cumulative toxicity range. Growth inhibition by 64 mmol/kg diet of CGT alone was marginal, varying from zero to 8%. In contrast, the combination of 1.0 mmol/kg of isotretinoin and 64 mmol/kg of CGT caused a reversible inhibition of tumour growth, culminating in a net decrease in tumour volume of 20%. This study documents the marginal enhancement of tumour growth by high sub-optimal concentrations of isotretinoin alone, and describes conditions for inhibition of tumour growth by sub-optimal concentrations of the natural retinoid. Related in vitro studies on retinoid sensitive and insensitive cell lines suggest that the anticancer activity of the combination is dependent on sensitivity of the cells to retinoids.