Imaging of dopamine D2 and somatostatin receptors in vivo using single-photon emission tomography in a patient with a TSH/PRL-producing pituitary macroadenoma

Diagnostic imaging of dopamine receptors in pituitary adenomas

Dopamine D2 receptor scintigraphy of pituitary adenomas is feasible by single-photon emission computed tomography using (123)I-S-(-)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-2-hydroxy-3-iodo-6-methoxybenzamide ((123)I-IBZM) and (123)I-epidepride. (123)I-epidepride is generally superior to (123)I-IBZM for the visualization of D2 receptors on pituitary macroadenomas. However, (123)I-IBZM and (123)I-epidepride scintigraphy are generally not useful to predict the response to dopaminergic treatment in pituitary tumour patients. These techniques might allow discrimination of non-functioning pituitary macroadenomas from other non-tumour pathologies in the sellar region. Dopamine D2 receptors on pituitary tumours can also be studied using positron emission tomography with (11)C-N-raclopride and (11)C-N-methylspiperone.

Enhanced prolactin levels in opium smokers

In Iran, opium is smoked for pleasure or as a medication by some people. It is a complex mixture of 40 different alkaloids, including morphine and codeine along with many impurities. Although it is well established that opioids or tobacco affect many physiological functions in humans, to our knowledge there has been no specific study looking at these effects in opium smokers. To assess that, we investigated the circulating levels of prolactin, TSH, LH, FSH and testosterone in male opium smokers who also smoke cigarettes (n=23, aged 28.4+/- 4.1 years), and comparing this with the corresponding values for nicotine abusers (n=12, 15-25 cigarettes/day) or a healthy control group (n=20) of the same age. Our results showed that 86.96% of the opium-dependent and 41.67 % of the nicotine-dependent group displayed high prolactin values (p<0.002). In addition, there was a positive correlation between the dose of opium and the plasma prolactin level of opium dependents (p=0.748, p<0.001). Low FSH was detected in 43.48% of the opium smokers and 50% of the cigarette smokers (p<0.001) with normal LH and testosterone levels. TSH of the opium smokers was also lower than that of the other two groups (p<0.002). In conclusion, the present data indicate that chronic opium and cigarette smoking may synergistically influence pituitary hormone production through the effects on neuropeptides produced either locally or systemic.

Pituitary tumor endocrinopathies and their endocrine evaluation

Pituitary tumors, depending on their respective cell type, manifest various endocrinopathies. Prolactinomas may present as hypogonadism and galactorrhea and can be diagnosed by measuring morning prolactin levels. Gonadotropinomas rarely cause gonadal hyperstimulation, and dynamic thyrotropin-releasing hormone stimulation testing is often required to elicit a diagnostic gonadotropin and/or subunit secretory response. Acromegaly is a multisystemic debilitating disease for which early diagnosis and treatment are crucial. Diagnostic criteria include a lack of plasma growth hormone suppression during the oral glucose tolerance test and elevation of age- and sex-matched insulin growth factor-1 levels. Patients harboring corticotropin-secreting adenomas characheristically present with signs and symptoms of hypercortisolism. Inferior petrosal sinus sampling for corticotropin may be required for microadenoma localization. Thyrotropinomas produce inappropriate thyrotropin (TSH) secretion and hyperthyroidism. The new third-generation TSH assay has improved the rate of detection of these lesions at an earlier stage.

Hormone levels and tumour size response to quinagolide and cabergoline in patients with prolactin-secreting and clinically non-functioning pituitary adenomas: predictive value of pituitary scintigraphy with 123I-methoxybenzamide

BACKGROUND

Dopamine agonists are indicated as primary therapy for PRL-secreting pituitary adenomas, while controversial results have been reported in nonfunctioning adenomas (NFA).

OBJECTIVE

To evaluate whether the in vivo visualization of dopamine D2 receptor expression detected by pituitary scintigraphy using 123I-methoxybenzamide (123I-IBZM) was correlated with the response to chronic treatment with quinagolide or cabergoline.

PATIENTS

10 patients affected with NFA (5 men and 5 women, age ranging between 25 and 50 years), and 10 with PRL-secreting naive macroadenomas (3 men and 7 women, age ranging between 22 and 59 years), serving as control.

STUDY DESIGN

All patients underwent an acute test with quinagolide: at 3-day intervals and in random order all patients received the drug (0.075 mg at 0800 h), or placebo. Blood samples were taken 15 and 5 minutes before and every 30 minutes for 6 h after drug or placebo administration. The test was considered positive when PRL and/or alpha-subunit levels decreased >/=50% as compared to baseline levels. After 6 months of treatment, 10 patients were randomised to continue the treatment with quinagolide and the remaining 10 received cabergoline for the remaining 6 months. The doses of quinagolide and cabergoline ranged from 0.075 to 0.6 mg/day and from 0.5 to 3 mg/week, respectively. At study entry, a magnetic resonance imaging (MR) study of the pituitary region and 123I-IBZM pituitary scintigraphy were performed. MR was repeated after 12 months of treatment to evaluate tumour shrinkage: reduction of tumour volume = 80% in prolactinomas and = 50% in NFA was considered significant. Basal PRL levels were 9495.0 +/- 1131.6 mU/l in prolactinomas and 602.4 +/- 50.5 mU/l in NFA.

RESULTS

The scintigraphy was negative in 6 out of 10 patients with NFA. Moderate uptake was observed in 3 patients with prolactinoma and 2 patients with NFA whereas intense uptake was observed in the remaining 7 patients with prolactinoma and 2 patients with NFA. Among the 8 patients with NFA and high circulating alpha-subunit levels, the acute test was negative in 5 while it was positive in the remaining 3 patients. The acute test was positive in all 10 patients with prolactinoma. After 12 months of treatment with quinagolide and cabergoline, circulating PRL levels were decreased in all 10 patients with prolactinoma (571.8 +/- 255.9 mU/l), being normalized in 7 patients. Suppression of PRL levels was found in all 10 patients with NFA (89.5 +/- 2.3 mU/l). A significant reduction of alpha-subunit levels was obtained in 9 out of 10 patients with NFA: in 4 out of 8 patients alpha-subunit levels were normalized. Significant adenoma shrinkage was recorded in 4 patients with prolactinoma among the 7 with intense pituitary uptake of 123I-IBZM. Significant adenoma shrinkage was recorded only in the 2 out of 10 patients with NFA with intense pituitary uptake of 123I-IBZM. A significant positive correlation was found between the degree of uptake (considered as score) and the response to quinagolide or cabergoline treatment (considered as percent hormone suppression) either in patients affected with PRL-secreting adenoma (r = 0.856, P < 0.005) or in those affected with NFA (r = 0.787, P < 0.05).

CONCLUSIONS

An intense 123I-IBZM uptake in patients with non-functioning adenomas was predictive of a good response to a chronic treatment with quinagolide and cabergoline. This result suggests that a pituitary 123I-IBZM scintigraphy could be considered in selected patients with non-functioning adenomas before starting medical treatment with dopamine agonists.

Cabergoline

Cabergoline (CAB) (1-[(6-allelylergolin-8 beta-yl)carbonyl]-1-[3-(dimethylamino)propyl]-3-ethyl-urea) is an ergoline derivative with potent, selective and long-lasting inhibitory activity on prolactin (PRL) secretion acting on dopamine receptors present in pituitary lactotrophes. Receptor binding studies have demonstrated that CAB has high in vitro selectivity and affinity for the subtype D2 of the dopamine receptor. In cultures of rat anterior pituitary cells, the concentrations of CAB and bromocriptine required to inhibit PRL secretory activity by 50% (IC50) were 0.1 and 3.4 nmol/l, respectively. As compared to bromocriptine, CAB was more potent in inhibiting the binding of [3H]N-n-propylnorapomorphine and it occupied the receptor for longer. These effects were observed in all areas of the rat brain. In vivo, CAB at doses of 0.125-1 mg twice weekly caused a dose-dependent suppression of PRL secretion in women with hyperprolactinaemia. CAB was shown to be significantly more effective than bromocriptine in inducing a complete biochemical response and clinical efficacy and was better tolerated than bromocriptine in the majority of patients. Notable tumour shrinkage until tumour disappearance was observed during CAB treatment in most patients with macroprolactinoma. CAB was also shown to be effective in patients resistant or poorly responsive to bromocriptine. In view of the limited data on CAB-associated pregnancies and the long half-life of the drug, it is currently recommended that women seeking to became pregnant, once ovulatory cycles have been established, should discontinue CAB therapy 1 month before they intend to conceive. However, no data on negative effects on pregnancy or offspring have been reported. The great efficacy of CAB together with its excellent tolerability makes this drug the current treatment of choice for the majority of patients with hyperprolactinaemic disorders. Very recently, the efficacy of CAB treatment has been reported in patients with acromegaly and clinically non-functioning adenomas with controversial results. CAB was also reported to have some efficacy in patients with Nelson's syndrome and Cushing's disease although these data are available only for limited case reports.

Receptor imaging with 111In-pentreotide and 123I-methoxybenzamide, and inhibition tests with octreotide and bromocriptine of mixed growth hormone/prolactin-secreting pituitary tumors

We have performed pituitary scintigraphy with 111In-pentreotide (OCT), a somatostatin analogue, and with metoxybenzamide (IBZM) by 123I-IBZM in two patients affected by mixed growth hormone/prolactin-secreting pituitary tumors. Short-term growth hormone (GH) inhibition by a single injection of OCT (100 micrograms s.c.), and short-term prolactin (PRL) inhibition by oral administration of 2.5 mg of bromocriptine (BCR), were also performed in both patients. The first patient, a 26 year old man, showed intense tumor uptake of 123I-IBZM scintigraphy, whereas 111In-OCT scintigraphy showed moderate tumor uptake. Five hours after the BCR inhibition test, a fall of 83% in PRL plasma levels (from 8,336 micrograms/L to 1,417 micrograms/L), and of 91.6% in GH plasma levels (from 39.5 micrograms/L to 3.3 micrograms/L) were observed. OCT inhibition test suppressed GH plasma levels from 36 micrograms/L to 3.5 micrograms/L. The patient was submitted to treatment with BCR and OCT. A dramatic shrinkage of the tumor was seen after six months of therapy. The lesion disappeared one year after the start of therapy. The second patient, a 64 year old man, showed intense uptake at 111In-OCT scintigraphy, while 123I-IBZM uptake was not observed. A test dose of BCR resulted in an acute fall of PRL (from 145 micrograms/L to 118 micrograms/L), but not of GH. A test dose of OCT decreased the GH plasma level from 61 micrograms/L to 4.5 micrograms/L. The patient was submitted to treatment with BCR and OCT that resulted in a computed tomography and magnetic resonance imaging decrease of 45% of tumor volume one year after the start of therapy. Our results suggest that both suppression tests with OCT and BCR, and scintigraphic studies in vivo with 123I-IBZM and 111In-OCT can be predictive for the effectiveness of therapies with dopamine agonists and/or SS-analogs in patients with mixed PRL/GH-secreting pituitary tumors. Further studies are required to evaluate the role of suppressive tests in selecting patients for appropriate clinical treatments.

Indium-111 pentetreotide single-photon emission tomography in patients with TSH-secreting pituitary adenomas: correlation with the effect of a single administration of octreotide on serum TSH levels

Few data are available on the visualization of somatostatin receptors in vivo in patients with thyrotropin (TSH)-secreting adenoma. We studied five patients with TSH-secreting adenomas using single-photon emission tomography (SPET) after administration of indium-111 pentetreotide. The intensity of 111In-pentetreotide uptake by the tumours was correlated with the degree of TSH suppression after a single administration of 100 microg octreotide s. c. Five patients (three women and two men) aged 27-46 years were investigated. Except for one patient with acromegaly, all had pure TSH-secreting tumours. One patient was previously untreated, while two had received octreotide, one antithyroid drugs, and one radioiodine. In all patients SPET demonstrated increased uptake of 111In-pentetreotide by the pituitary adenoma. The target to non-target ratio (T/nT) of 111In-pentetreotide uptake was higher than 10 in three patients. Administration of 100 microg octreotide s. c. caused a significant reduction in TSH levels from 4.8+/-1.4 mU/l to a nadir of 3.1+/-1.1 mU/l after 6 h (P<<0.001 by ANOVA). Suppression of TSH secretion ranged from 30% to 60% of the baseline value. The T/nT ratio showed a trend toward a direct relationship with the degree of TSH inhibition after acute octreotide administration (r=0.67; P=NS). Our study showed that 111In-pentetreotide scan visualized somatostatin receptors in all five of the patients with TSH-secreting pituitary adenomas, confirming the frequent presence of somatostatin receptors in these rare tumours, even though the correlation with the TSH inhibition after a single administration of octreotide did not reach significance.

Diagnosis and management of nonfunctioning pituitary tumors

It is now recognized that most clinically nonfunctioning pituitary tumors express gonadotropin hormones or their subunits in vitro, and sometimes in vivo. Many of these tumors are diagnosed by sensitive imaging techniques while patients are still asymptomatic. We outline an overview of the diagnosis and clinical management of these tumors.

Imaging of pituitary tumours

In the neuroradiological study of pituitary tumours, second generation CT, dynamic CT and MRI provide information about the extent of the tumour and its anatomical relations with the surrounding tissues. Sometimes these techniques can distinguish primary anterior pituitary lesions from primary parasellar lesions with presentations in the sellar region. In general, contrast-enhanced MRI and dynamic CT are more sensitive than conventional CT for the diagnosis of pituitary microadenomas, as well as for the precise delineation of the parasellar invasion of macroadenomas. Radiological techniques usually cannot distinguish clinically non-functioning from functioning pituitary adenomas. BSIPSS is used for confirmation of the diagnosis of Cushing's disease as well as for the lateralization of pituitary microadenomas in Cushing's disease and some other anterior pituitary hyperfunctional states. Neurotransmitter-receptor ligand imaging by SPECT with 123I-IBZM and/or 111In-DTPA-octreotide characterizes the dopamine D2 and somatostatin receptor status of pituitary adenomas, respectively. In selected cases, these techniques may be used for the differential diagnosis of pituitary tumours as well as for the differential diagnosis of primary anterior pituitary lesions and primary parasellar lesions with presentations in the sellar region. If medical therapy of these tumours with receptor agonists is being considered, these techniques can help in selecting the first-line treatment. Furthermore, the effects of medical therapy on the tumour can be evaluated. The introduction of newer and more receptor-specific radioligands may expand the clinical use of these techniques in the future. The availability of PET for the clinical diagnosis of pituitary tumours is still limited, but promising results have been described.