ORG-2766, AN ACTH(4-9)-MSH(4-9) ANALOG, MODULATES VINCRISTINE-INDUCEDNEUROTOXICITY IN THE SNAIL LYMNAEA-STAGNALIS

In vincristine (VCR) chemotherapy the dose-limiting factor is neurotox icity, a side-effect which probably is caused by the action of the dru g on axonal microtubules. VCR transforms them into paracrystalline str uctures. ORG 2766, an ACTH(4-9)/MSH(4-9), analogue, might have protect ive properties against this toxicity, because of its potency to induce the formation of microtubules. Therefore, the effects were studied of co-treatment with VCR and ORG 2766 on paracrystals and on microtubule s in axons of the cerebral commissure (CC) of the snail Lymnaea stagna lis. In vitro experiments were carried out, incubating cerebral gangli a including the CC, for various periods of time (5-25 h) in Ringer's s olution, in ORG 2766 (10(-6) M), in VCR (10(-4) M), or in VCR+ORG 2766 . Furthermore, the effects of VCR (5 h) were studied in CC, pre- and/o r post-treated with ORG 2766 (10 h). In another experiment VCR was use d at a concentration at which no paracrystals were formed (2x10(-6) M) . In this experiment VCR-treatment (5, 15 h) was compared to pre treat ment and pre- and post-treatment with ORG 2766 (10 h). The numbers of microtubules (per mu m(2) axoplasm) and paracrystals (number encounter ed along perpendicular axes of the CC) were counted and the size of th e paracrystals was measured in cross-sections of the CC, using electro n microscopy. Irrespective of the type of additional treatment, signif icant differences were not observed in the numbers of paracrystals (32 +/-10) after VCR-treatment for 5 h. Between 5 and 10 h of incubation t he number increased (to 55+/-21) and remained approximately at this le vel. The number of microtubules decreased during the first 5 h of VCR treatment from 128+/-16 (control) to 37+/-10, and decreased further 19 +/-5 between 5 and 10 h of incubation. The size of the paracrystals of all groups treated for 5 h with either VCR or VCR+ORG 2766 were simil ar (x=1.7 mu m(2)) However, the paracrystals were significantly larger in all groups co-treated for longer periods (10, 15 or 20 h) with ORG 2766 (VCR: x=2.3-3.0 mu m(2); VCR+ORG 2766: x=3.0-3.4 mu m(2)). This indicates that after an initial period of formation of paracrystals, t ubulin is added to the existing paracrystals. At the low VCR concentra tion studied no paracrystals were formed. However, the number of micro tubules had decreased significantly (from 159+/-10 to 84+/-17 after 15 h treatment). Pre- as well as pre- and post-treatment with ORG 2766 r esulted in microtubule numbers which were not different from control v alues, indicating the positive effect of ORG 2766. In the groups pre- and/or post-treated with the high concentration of ORG 2766 (especiall y in the pre-treated group), the remaining number of microtubules was higher than in those co-treated with VCR and ORG 2766, but lower than in the controls. Although translation of the present results into clin ical terms is difficult, they suggest that pre-and/or post-treatment w ith ORG 2766 may have a benificial effect on VCR-induced neurotoxicity . This conclusion is based on the observation that when ORG 2766 is gi ven before VCR administration or after VCR clearance, larger numbers o f microtubules were found in the nervous tissue than after treatment w ith VCR alone.