Prevention of drug-induced upset to energy production in the S180 sarcoma by inhibition of prostaglandin synthetase [proceedings

Abstract

In theory, interference with energy production in tumours with the non-cytotoxic drugs L-isoproterenol (Jones et al., 1979) and hydralazine (G. R. N. Jones, unpublished work; Jones et al., 1980) offers an attractive means of causing selective cancer destruction in vivo. The fall in the total adenine nucleotide content of the tumour mass has provided a useful index of necrosis in preliminary studies (Jones et al., 1978). In practice, however, the results of repeated treatments in adult Swiss mice bearing the solid S180 sarcoma have been a little disappointing. For example, even though growth was cut by 65%, total adenine nucleotide concentrations of the tumour were 0.9 f 0.3 and 1.1 f 0.5 nmol/mg wet wt. after treatment with hydralazine ( 5 m g h g daily, subcutaneously) for 2 and 4 days respectively (controls, 2.3 f 0.4 nmol/mg); these are higher values than those obtained 24 h after one single administration (G. R. N. Jones, unpublished work; Jones et al., 1980). On the other hand, acquisition of resistance to continuing anti-cancer therapy is also seen in other forms of management (e.g. Boesen & Davis, 1969; Brule et al., 1973; Harrap, 1976). The emergence of this refractory state can be explained in terms of the depletion of limited amounts of an endogenous factor involved in disruption of energy-yielding processes. Alterations in the tumour adenine nucleotide pattern brought about by hydralazine are basically similar to those produced by endotoxin (Jones et al., 1978), where uncoupling of oxidative phosphorylation is followed by progressive injury in tumour mitochondria (Jones, 1979). Differences exist insofar as (a) drug-induced interference with energy production occurs more rapidly in response to either drug than to endotoxin, and (b) the energy charge (Atkinson & Walton, 1967) gradually returns to normal ir, the original tumour mass after drug treatment but not after endotoxin (Jones et al., 1980). Various explanations of tumour necrotization by endotoxin have been suggested (Jones, 1979), including the sequence (a) selective activation of phospholipase A, in tumour mitochondria, (b) local hydrolysis of phosphatides to lysophosphatides and fatty acids, including arachidonic acid, and (c) enzymic conversion of the latter to prostaglandins. Interference with oxidative phosphorylation could then arise both through prostaglandins acting as Caz+ ionophores (Kirtland & Baum, 1972; Malmstrom & Carafoli, 1975), and through lysophosphatides acting as uncoupling agents (Witter et al., 1957). Only the first of these mechanisms is indomethacin-sensitive. Two lines of evidence suggest involvement of arachidonic acid or its metabolites in drug-induced tumour injury. First, pretreatment of Swiss mice bearing the S 180 sarcoma in solid form with indomethacin ( 5 mg/kg, intraperitoneally) may largely prevent changes in the tumour adenine nucleotide pattern brought about by hydralazine or L-isoproterenol at 1 h (Table 1). Similar effects were seen when animals given L-isoproterenol had been pretreated with the prostaglandin synthetase inhibitors flurbiprofen (Boots) or Voltaren (CIBA; results not shown). Changes in the tumour adenine nucleotide pattern 4 h after intraperitoneal injection of endotoxin or the powerful tumour-promoter 12-0-tetradecanoylphorbol 13-acetate (Hecker et al., 1964) are also presented. In contrast, these responses were affected by indomethacin pretreatment only a little or not at all over 4 h (Table 1 ). Second, arachidonic acid itself also produced effects. Tumour-bearing mice (about 30g) were given the free acid (9096 pure; approx. 15 mg/kg, intraperitoneally) in olive oil (0.05 mi), and were killed 25-47 h later. The average energy charge from a total of 12 tumours was 0.69f0.036, which is well below the usual value (Table 1; see also Jones et al., 1980). In another animal, injected daily for 3 days and killed on the fourth, the tumour was overtly haemorrhagic and necrotic, while the total adenine nucleotide content, at 1.3 nmol/mg, had fallen to approximately half the control value. These preliminary data are fully consistent with the depletion concept outlined above. On this basis both the capacity of cellular phospholipids to provide prostaglandin precursors and the net yield of functional Ca2+ ionophores could influence the magnitude of the response of the tumour energy charge to hydralazine or L-isoproterenol. Since such normal tissues as liver (Lankin, 1973) and serum (Elmendorff, 1965) tend to be depleted of arachidonic acid in tumour-bearing animals and patients, a lower content of arachidonic acid might be anticipated in the re-acylated lysophosphatides of tumour cells surviving the initial treatment. Both this effect and a similar deficiency in daughter generations could explain the development of refractoriness to continued therapy.