Clinical Pharmacology and Therapeutics
A Textbook of Clinical Pharmacology and ... - clinicalevidence
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318 THE PITUITARY HORMONES AND RELATED DRUGS<br />
<strong>and</strong> thereby reduce pituitary stimulation of male or female<br />
gonads, effectively leading to medical orchidectomy/ovariectomy<br />
(a state of hypopituitary hypogonadism).<br />
Adverse effects<br />
Menopausal symptoms are common in addition to decreased<br />
trabecular bone density, <strong>and</strong> local symptoms caused by irritation<br />
of the nasal mucosa with buserelin.<br />
Pharmacokinetics<br />
Gonadorelin analogues are peptides <strong>and</strong> are given parenterally.<br />
Goserelin may be given as intravenous pulses to<br />
mimic the physiological release of GnRH. Depot preparations<br />
are available to suppress FSH/LH release (see above). GnRH<br />
analogues are cleared by a combination of hepatic metabolism<br />
<strong>and</strong> renal excretion.<br />
Key points<br />
Gonadotrophins <strong>and</strong> GnRH analogues<br />
• FSH <strong>and</strong> LH are secreted in pulses <strong>and</strong> stimulate<br />
gonadal steroid synthesis.<br />
• GnRH analogues initially stimulate, but then<br />
downregulate the release of FSH <strong>and</strong> LH.<br />
• GnRH analogues (e.g. goserelin, buserelin) are used in<br />
the treatment of :<br />
– endometriosis;<br />
– female infertility;<br />
– prostate cancer;<br />
– advanced breast cancer.<br />
• Side-effects of GnRH analogues include:<br />
– menopausal symptoms;<br />
– reduced bone density (by reducing oestrogen secretion).<br />
relation to the treatment of diabetes insipidus. Demeclocycline<br />
is an antagonist of ADH <strong>and</strong> has been used in patients with<br />
hyponatraemia caused by the syndrome of inappropriate ADH<br />
secretion (SIADH). More specific antagonists of ADH are in<br />
development. The use of oxytocin for induction of labour is<br />
described in Chapter 41.<br />
Key points<br />
Physiology of the pituitary<br />
Anterior pituitary<br />
secretes:<br />
• growth hormone (GH);<br />
• follicle-stimulating hormone (FSH);<br />
• luteinizing hormone (LH);<br />
• adrenocorticotrophic hormone (ACTH);<br />
• prolactin;<br />
• thyroid-stimulating hormone (TSH).<br />
is controlled by:<br />
• hypothalamic hormones (stimulatory/inhibitory);<br />
• feedback inhibition.<br />
Posterior pituitary<br />
Related octapeptides, synthesized in the hypothalamus<br />
<strong>and</strong> released by neurosecretion:<br />
1. Vasopressin (ADH):<br />
– increases blood pressure;<br />
– causes renal water retention.<br />
2. Oxytocin:<br />
– stimulates uterine contractions;<br />
– used in obstretrics (for induction of labour).<br />
Case history<br />
ADRENOCORTICOTROPHIC HORMONE<br />
Adrenocorticotrophic hormone (ACTH, corticotropin) is no<br />
longer commercially available in the UK. A synthetic analogue<br />
of ACTH, containing only the first 24 amino acids, is available as<br />
tetracosactide. This possesses full biological activity, the remaining<br />
15 amino acids of ACTH being species specific <strong>and</strong> associated<br />
with antigenic activity. The t 1/2 of tetracosactide (15 minutes)<br />
is slightly longer than that of ACTH, but otherwise its properties<br />
are identical. Tetracosactide is used as a diagnostic test in the<br />
evaluation of patients in whom Addison’s disease (adrenal insufficiency)<br />
is suspected. A single intravenous or intramuscular dose<br />
is administered, followed by venous blood sampling for plasma<br />
cortisol determination. There is a small risk of anaphylaxis.<br />
POSTERIOR PITUITARY HORMONES<br />
Vasopressin (antidiuretic hormone, ADH) <strong>and</strong> oxytocin are<br />
related octapeptide hormones synthesized in the supra-optic<br />
<strong>and</strong> paraventricular hypothalamic nuclei <strong>and</strong> transported<br />
along nerve fibres to the posterior lobe of the pituitary gl<strong>and</strong><br />
for storage <strong>and</strong> subsequent release (neurosecretion). Vasopressin<br />
<strong>and</strong> desmopressin (DDAVP) are discussed in Chapter 36, in<br />
A 64-year-old man was investigated for worsening chronic<br />
back pain <strong>and</strong> was found to have osteosclerotic bony metastases<br />
from prostate carcinoma. Analgesia with adequate<br />
doses of NSAIDs successfully controlled his bone pain, <strong>and</strong><br />
he was started on GnRH analogue therapy with goserelin<br />
given subcutaneously, 3.6 mg per month. After one week<br />
his pain was worse, especially at night, without evidence of<br />
spinal compression.<br />
Question<br />
What is the likely cause of the deterioration in his symptoms<br />
<strong>and</strong> how would you treat him?<br />
Answer<br />
The most likely cause of his symptoms worsening in the first<br />
week of GnRH analogue therapy is the ‘tumour flare reaction’.<br />
GnRH analogues increase secretion of FSH/LH for one<br />
to two weeks, causing an initial increase in testosterone.<br />
They subsequently produce downregulation, leading to<br />
decreased secretion of FSH/LH <strong>and</strong> hence decreased testosterone<br />
levels. In patients with metastatic prostate cancer it<br />
is essential to initiate GnRH analogue therapy only after<br />
several weeks of treatment with an <strong>and</strong>rogen receptor<br />
antagonist such as cyproterone acetate, flutamide or bicalutamide.<br />
The use of anti-<strong>and</strong>rogens prevents the ‘tumour<br />
flare’. Thus this patient should be given adequate analgesia<br />
<strong>and</strong> an <strong>and</strong>rogen receptor antagonist (e.g. oral flutamide)<br />
started at once. Goserelin can then be restarted in<br />
several weeks time.