Phenotype and resistance patterns of 10 resistant prolactinomas

Dopamine agonists (DA) are the first-line therapy in prolactinomas, but they fail to decrease prolactin (PRL) levels and/or tumor size in some of these tumors, which are labeled as resistant prolactinomas (RP). To date, risk factors for DA resistance are not fully understood and management of DA-RP is not well established.

We retrospectively recorded clinical, biochemical and radiological features, as well as management and outcome, of all cabergoline (CAB)-RP attended at the Endocrinology department of a tertiary hospital between 1995 and 2016. CAB resistance was defined as the failure to normalize PRL (biochemical resistance, BR) or reduce tumor size by at least 50% (morphological resistance, MR) with a CAB dose up to 2mg/week (or 3mg/week in cases where lower doses were not tested) for at least 3 months.

Ten CAB-RP were found. The mean age of the cohort was 30.6 years and 50% of subjects were male. The average tumor size was 1.78cm (80% macroadenomas). The mean maximal dose of CAB was 3.8mg/week. Five patients showed isolated MR, four combined MR+BR and only one isolated BR. MR patients were more often males and older than MR+BR patients. Transsphenoidal surgery achieved normalization of PRL and/or disappearance of tumor in three of seven patients. At the end of follow up all patients had controlled PRL levels (with or without CAB) and most of them bore a visible although stable tumor.

Isolated MR and combined MR+BR are the most frequent patterns of DA resistance whereas isolated BR seems to be uncommon. Our data support a high tumor size but not male gender as a risk factor for DA resistance.