Fosamprenavir calcium

Telzir (GlaxoSmithKline)
700 mg tablets
225 mL bottles containing 50 mg/mL suspension
Approved indication: HIV infection
Australian Medicines Handbook section 5.4.3

Amprenavir is a protease inhibitor that can be used in combination with other drugs to treat patients infected with HIV.¹ It can be given with ritonavir as their interaction significantly increases the plasma concentration of amprenavir (see New drugs, Aust Prescr 2002;25:44-7).

Fosamprenavir has been developed to try and overcome some of the pharmacological disadvantages of amprenavir, such as low solubility. It is a prodrug which is rapidly converted to amprenavir during absorption. Fosamprenavir can also be given with low dose ritonavir to increase the concentration of amprenavir.

Fosamprenavir was compared with nelfinavir in a study of patients who had not previously received antiretroviral drugs. Each group of patients also received abacavir and lamivudine. After 48 weeks 66% of the 166 patients given fosamprenavir and 51% of the 83 patients given nelfinavir had less than 400 copies of viral RNA/mL. The median increase in CD4 cell counts was 201 cells/mm3 in the fosamprenavir group and 216 cells/mm3 in the nelfinavir group.²

Another study enrolled 315 patients who had previously been treated with protease inhibitors. This compared regimens of fosamprenavir and ritonavir to a regimen of lopinavir and ritonavir. After 48 weeks 58% of the patients taking fosamprenavir twice daily had less than 400 copies of viral RNA/mL. However, 61% of the patients taking lopinavir with ritonavir had the same response. It is therefore uncertain that the fosamprenavir regimens are as effective as lopinavir with ritonavir.

The adverse effects and interactions of fosamprenavir are the same as those of amprenavir. Gastrointestinal symptoms and skin rashes are common. As fosamprenavir is metabolised by P450 3A4 it must not be prescribed with drugs such as ergot derivatives, midazolam or triazolam. Fosamprenavir can also interact with complementary medicines such as St John's wort.

Although the pharmacology of fosamprenavir may be an advance on amprenavir, there is currently no evidence to show this will improve the clinical outcomes for patients. Fosamprenavir is only approved for use in combination with ritonavir.

Some of the views expressed in the following notes on newly approved products should be regarded as preliminary, as there may have been limited published data at the time of publication, and little experience in Australia of their safety or efficacy. However, the Editorial Executive Committee believes that comments made in good faith at an early stage may still be of value. Before new drugs are prescribed, the Committee believes it is important that more detailed information is obtained from the manufacturer's approved product information, a drug information centre or some other appropriate source.