Prolactinomas Resistant to Dopamine Agonists: Pathophysiology and Treatment

Prolactinomas are the most common functional pituitary tumors, accounting for 40% of all pituitary adenomas. Medical treatment with dopamine agonists (DA), mainly cabergoline, is considered the primary therapy for these patients. Prolactin normalization is achieved in 80–90% of prolactinomas treated with cabergoline. Patients resistant to the standard dose can escalate the dose of cabergoline up to the maximum tolerated dose. The expression of dopamine (D2) receptors and dopamine affinity is decreased in aggressive and resistant prolactinomas. Patients with aggressive and DA-resistant adenomas or with rare PRL-secreting carcinomas can be treated off-label with temozolomide (TMZ), a DNA alkylating agent. TMZ is effective in 40–50% of treated lactotroph tumors showing at least a partial response. However, patients tend to escape from the effect of TMZ after a limited time of response. Other therapeutic options include aromatase inhibitors, the somatostatin receptor ligand pasireotide, peptide receptor radionuclide therapy (PRRT), immune-checkpoint inhibitors, tyrosine-kinase inhibitors, or everolimus, the mammalian target of rapamycin inhibitor. These experimental treatments were effective in some patients carrying refractory prolactinomas showing usually partial tumor control. However, the number of treated patients with any of these new therapeutic options is very limited and treatment results are inconsistent, thus additional experience with more patients is required.