Cabergoline Therapy of Growth Hormone & Growth Hormone/Prolactin Secreting Pituitary Tumors

Dopamine agonists have been used as adjunctive therapy for acromegaly for many years, but relatively few studies have assessed the efficacy of a newer agonist, cabergoline. Some data suggest that cabergoline may be more effective than bromocriptine, in particular for those patients whose tumors secrete both growth hormone and prolactin. In order to assess this possibility further, we have evaluated the biochemical response to cabergoline therapy in patients with acromegaly at our center. We describe first an unusual patient who presented with a pituitary macroadenoma secreting both GH and prolactin. At presentation he had elevated levels of growth hormone 6.0 microg/L, IGF-I, 722 ng/ml, and prolactin, 6000 ng/ml. Cabergoline therapy alone was highly effective in this patient and normalized his levels of all three hormones and his gonadal function as well as produced significant shrinkage of his pituitary tumor. Fourteen other patients with more typical, active postoperative acromegaly were administered cabergoline in a 6-month, open label, dose-escalation study. Mean baseline GH was 1.3 +/- .23 ng/ml and fell to a nadir of 0.85 +/- .18 ng/ml on cabergoline therapy (p = 0.03). Mean baseline IGF-I was 520 +/- 45.2 ng/ml and fell to a mean nadir during cabergoline therapy of 368 +/- 29.8 ng/ml (p = 0.0013). At the completion of the cabergoline therapy study period, however, mean IGF-I was 453 +/- 46 ng/ml, not significantly lower than the baseline value (p = 0.11). No changes in tumor sizes occurred on cabergoline therapy. Eight of 14 patients achieved a normal IGF-I at some point during the 24 weeks study period, but the efficacy of cabergoline waned with time as only 3 of 14 (21%) of patients had a persistently normal IGF-I with up to 18 months of cabergoline therapy. Six patients had modest hyperprolactinemia at diagnosis (26-142 ng/ml) and 5 patients had positive immunohistochemical staining of their tumor for prolactin, but in neither of these small groups was cabergoline therapy more effective at normalizing IGF-I than in those patients with apparently pure GH secreting tumors. Three of 14 patients (21%) had side effects that limited therapy. A trial of cabergoline as adjunctive therapy may be considered in select patients with mild disease and small tumor residuals, but the expectation for biochemical control in these patients needs to be kept low, even for tumors that co-secrete GH and prolactin.