Inhibitory effects of the dopamine agonists quinagolide (CV 205-502) and bromocriptine on prolactin secretion and growth of SMtTW pituitary tumors in the rat

The SMtTW tumor, a spontaneous PRL-secreting transplantable tumor, is the only available animal model sensitive to dopamine agonists. This model has been used to compare the long term in vivo effects of CV 205-502 (CV) and bromocriptine (BR) on PRL secretion and tumor growth. These two drugs were given for 2 months to female Wistar-Furth rats bearing either small or large tumors 4 and 6 months after the graft. Untreated grafted rats served as control. In all rats treated with 5 or 10 mg/kg.day BR or 0.3 mg/kg.day CV, a normalization of plasma PRL levels was observed whatever the pretreatment levels (plasma PRL or CV or BR-treated rats, < 15 ng/ml vs. 28253 ng/ml in control rats 8 months after graft). An inhibition of tumor growth was found for both small and large tumors, but the tumors never disappeared completely (mean tumor weights at autopsy, 440 and 660 mg in BR and CV groups vs. 5270 mg in control group 8 months after graft). Experiments performed with increasing doses of BR (0.15-5 mg/kg.day) or CV (0.03-0.6 mg/kg.day) indicated that CV is effective at doses 5-10 times lower than those of BR. A shrinkage under treatment and a regrowth after drug withdrawal were demonstrated for large tumors by in vivo ultrasonographic measurements of tumor size. Histological and ultrastructural effects were similar for the two drugs: decrease in hemorrhage, reduction of the cell size and secretory activity, increase in immunoreactive PRL cellular content, and inhibition of exocytosis. There was no difference in the PRL mRNA content of treated and untreated tumors, as assessed by in situ hybridization. In conclusion, CV and BR exhibit similar inhibitory effects on tumor growth and PRL secretion. These effects are rapidly and fully reversible after drug withdrawal. The present results give a complete account of the actions of the two dopamine agonists under conditions comparable to those used in the treatment of human prolactinomas.