Apomorphine

Apomorphine is a type of dopaminergic agonist, a morphine derivative (but does not actually contain morphine, or bind to opioid receptors). Apomorphine is a relatively non-selective dopamine receptor agonist, having possible slightly higher affinity for D₂-like dopamine receptors.

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Historically, apomorphine has been tried for a variety of uses including psychiatric treatment of homosexuality in the early 20th century. Currently, apomorphine is used in the treatment of Parkinson's disease and (under the name Uprima) erectile dysfunction. It was also successfully used in the treatment of heroin addiction, a purpose for which it was championed by the author William S. Burroughs. It is a potent emetic, meaning that it should not be administered without an antiemetic such as domperidone. The emetic properties of apomorphine are exploited in veterinary medicine to induce therapeutic emesis in canines that have recently ingested toxic or foreign substances.

For treatment of erectile dysfunction, it is believed that dopamine receptors in the hypothalamic region of the brain are the main target, as although dopamine receptors in the penis do facilitate erection, they do so far more weakly than those in the brain.^[1]

Apomorphine is clear as a liquid but stains green. Therefore care must be taken to avoid splashes. Apormophine does not remain stable for more than 24 hours in a plastic container, so syringes are discarded if not used within 24 hours.

Use in Parkinson's disease

First mooted as a treatment for Parkinson's disease as early as 1951,^[2] its clinical use was first reported in 1970 by Cotzias et al,^[3] although its emetic properties and short half-life made oral use impractical. A later study found that combining the drug with the antiemetic domperidone improved results significantly.^[4]

Therapeutic use in Parkinson's disease is effective because of the drugs strong dopaminergic action, with a rapid effect (within 3-20 minutes of injection) but a brief duration.^[5] Whilst apomorphine can be used in combination with l-dopa, the intention is usually to wean patients off of this, as by this stage they will probably be experiencing a great deal of dopa-induced dyskinesias and "off" periods.^[5] Following a successful apomorphine challenge, training of patient and caregiver, and careful dose titration, there is no reason why an apomorphine pump can not be an effective monotherapy.^[5]

Erectile dysfunction

Apomorphine hydrochloride (trade name "Uprima") is used in the treatment of erectile dysfunction (male impotence). Its mode of stimulating dopamine in the brain which is believed to enhance the sexual response. It was found to be of poor efficacy^[6] in a large-scale study by Researchers at the UK's Drug Safety Research Unit and University of Portsmouth and discontinued in the UK in January 2006.^[6] Around 65-70% of doctors felt it was ineffective, with 60% of over 11,000 patients (avg age 61) discontinuing in month 1 and a further 23% in month 2.^{[6] [7]} Nonetheless some sources continue to supply the drug claiming it is effective.^[8]

References

1. ^ Matsumoto K, Yoshida M, Andersson K, Hedlund P (2005). "Effects in vitro and in vivo by apomorphine in the rat corpus cavernosum.". Br J Pharmacol 146 (2): 259-67. PMID 16025145.
2. ^ Schwab R, Amador L, Lettvin J. "Apomorphine in Parkinson's disease.". Trans Am Neurol Assoc 56: 251-3. PMID 14913646.
3. ^ Cotzias G, Papavasiliou P, Fehling C, Kaufman B, Mena I (1970). "Similarities between neurologic effects of L-dopa and of apomorphine.". N Engl J Med 282 (1): 31-3. PMID 4901383.
4. ^ Corsini G, Del Zompo M, Gessa G, Mangoni A (1979). "Therapeutic efficacy of apomorphine combined with an extracerebral inhibitor of dopamine receptors in Parkinson's disease.". Lancet 1 (8123): 954-6. PMID 87620.
5. ^ ^{a b c} Chaudhuri K, Clough C (1998). "Subcutaneous apomorphine in Parkinson's disease.". BMJ 316 (7132): 641. PMID 9522772.
6. ^ ^{a b c} Pharmaceutical Business Review, "Study shows Abbott's Uprima ineffective for most UK patients"
7. ^ MedicineNet study review
8. ^ For example: http://www.healthexpress.co.uk/Uprima.cfm