Several medicines can be used to treat pituitary tumors that are making hormones.
Drugs for prolactin-secreting tumors (prolactinomas)
Drugs called dopamine agonists, such as bromocriptine (Parlodel®) and cabergoline, are so effective in both blocking prolactin production by prolactinomas and preventing growth of these tumors that surgery is usually not necessary. Cabergoline has an advantage of lasting longer, so it does not need to be taken as often. Another plus is cabergoline may have fewer side effects.
Most patients are able to control their prolactin levels with medicine. The drugs are also very effective in reducing the size of prolactin-secreting macroadenomas. Most of these tumors shrink to less than half of their original size after treatment. Only 15% to 20% of the tumors do not shrink at all after treatment. Even if the tumor doesn't shrink, these drugs often can prevent prolactinomas from growing larger. If successful, the drug treatment may be continued for life.
Possible side effects of these drugs include drowsiness, dizziness, nausea, vomiting, diarrhea or constipation, confusion, and depression. Another “side effect” is that these drugs may restore fertility in women whose high prolactin levels had been causing infertility. Cabergoline may also increase the risk of heart valve problems, but this is rare in patients taking this drug for prolactinomas.
Drugs for growth hormone-secreting tumors
Octreotide (Sandostatin®) is a modified form of the natural hormone somatostatin. Somatostatin is made in the pituitary and other glands and was named for its ability to inhibit growth hormone (somatotropin) secretion. This medicine blocks growth hormone production by adenomas and returns insulin-like growth factor-1 (IGF-1) to normal levels in about two thirds of patients. It is first given as an injection under the skin 3 times per day. A longer acting form is available, which can be given as a monthly injection. A newer, similar drug called lanreotide (Somatuline®) is given as an injection every 1 to 2 weeks or every month. The effectiveness of these drugs is measured by testing blood growth hormone and IGF-1 levels. Tumors tend to shrink very slowly with these drugs.
Both drugs can have minor side effects, such as nausea, vomiting, diarrhea, stomach pain, dizziness, headache, and pain at the site of injection. Many of these side effects improve or even go away with time. They can also cause gallstones and may worsen diabetes if a person has it.
Dopamine agonists such as bromocriptine or cabergoline may reduce growth hormone levels in about 20% of patients. Unfortunately, higher doses are needed to be effective for these tumors than for prolactinomas, and some patients have trouble tolerating the side effects they can cause (discussed above). The advantage of these drugs is that they can be taken as a pill.
If these drugs are not effective, pegvisomant (Somavert®), which works by blocking the action of growth hormone on other cells, can be used. It is very effective in lowering blood IGF-1 levels, but it doesn't block growth hormone secretion by the pituitary gland or shrink pituitary tumors. It has few side effects, except that it may cause mild liver damage in some people. It is given by daily injection under the skin.
Drugs for corticotropin (ACTH)-secreting tumors
Medicines are not usually part of the treatment of these tumors unless surgery and radiation therapy don't work. Cyproheptadine (Periactin®) suppresses ACTH production in about half of these tumors. For patients who do not respond to cyproheptadine, several drugs can be used to keep the adrenal gland from making cortisol. These include ketoconazole, aminoglutethimide, etomidate, metyrapone, and mitotane. These drugs can be hard to take because of side effects, but they can have a clear benefit when surgery is not an option.
Drugs to treat thyrotropin (TSH)-secreting tumors
For these tumors, drugs such as octreotide and lanreotide can usually reduce the amount of TSH that is produced. Bromocriptine or cabergoline can also be used. These drugs are discussed in more detail above.
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