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.