Protease inhibitor (amprenavir prodrug) Pregnancy: US labelling §8.1: limited data are available for use of fosamprenavir in pregnancy, and there are insufficient human data to adequately assess a drug-associated risk for birth defects and miscarriage - given the limited number of exposed pregnancies, no conclusions can be drawn on the safety of fosamprenavir in pregnancy. Fosamprenavir 700 mg twice daily with ritonavir 100 mg twice daily should only be considered in pregnant patients who are already on a stable twice-daily fosamprenavir/ritonavir 700/100 regimen prior to pregnancy and who are virologically suppressed (HIV-1 RNA less than 50 copies/mL). A pregnancy exposure registry (the Antiretroviral Pregnancy Registry, 1-800-258-4263) monitors outcomes in women exposed to fosamprenavir during pregnancy.

Fosamprenavir

Brand names: Telzir

Fosamprenavir is a prodrug HIV protease inhibitor used, with low-dose ritonavir boosting, as part of combination antiretroviral therapy for HIV-1 infection.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Therapy-naive adults: fosamprenavir 1,400 mg twice daily (without ritonavir); OR fosamprenavir 1,400 mg once daily plus ritonavir 200 mg once daily; OR fosamprenavir 1,400 mg once daily plus ritonavir 100 mg once daily; OR fosamprenavir 700 mg twice daily plus ritonavir 100 mg twice daily

Route: Oral - may be taken with or without food

Frequency: Twice daily or once daily depending on which of the listed regimens is selected

US prescribing information §2.2 (Fosamprenavir Calcium Tablets, 700 mg). No UK SPC was captured in this bundle, so this is US labelling and must be verified against the UK SPC. PROTEASE INHIBITOR-EXPERIENCED ADULTS: fosamprenavir 700 mg twice daily plus ritonavir 100 mg twice daily (the once-daily regimens are listed only for therapy-naive adults). PREGNANCY: fosamprenavir 700 mg twice daily plus ritonavir 100 mg twice daily should only be considered in patients who are already on a stable twice-daily fosamprenavir/ritonavir 700 mg/100 mg regimen prior to pregnancy and who are virologically suppressed (HIV-1 RNA less than 50 copies/mL); lower amprenavir exposures occur during pregnancy so viral load should be monitored closely, and data on other fosamprenavir regimens in pregnancy are not available. Higher-than-approved dose combinations of fosamprenavir plus ritonavir are NOT recommended because of an increased risk of transaminase elevations. When fosamprenavir is used with ritonavir, consult the full ritonavir prescribing information. HEPATIC IMPAIRMENT (§2.4) - use with caution at reduced dosage: MILD (Child-Pugh 5-6) 700 mg twice daily without ritonavir (therapy-naive) or 700 mg twice daily plus ritonavir 100 mg once daily (therapy-naive or PI-experienced); MODERATE (Child-Pugh 7-9) 700 mg twice daily without ritonavir (therapy-naive) or 450 mg twice daily plus ritonavir 100 mg once daily; SEVERE (Child-Pugh 10-15) 350 mg twice daily without ritonavir (therapy-naive) or 300 mg twice daily plus ritonavir 100 mg once daily. SULFONAMIDE ALLERGY: use with caution in patients with a known sulfonamide allergy (§5.3). PAEDIATRIC (§2.3, at least 4 weeks to 18 years) - dosing is calculated on body weight and must not exceed the recommended adult dose; the twice-daily oral-suspension regimens with concurrent ritonavir are: under 11 kg, fosamprenavir 45 mg/kg plus ritonavir 7 mg/kg; 11 to under 15 kg, fosamprenavir 30 mg/kg plus ritonavir 3 mg/kg; 15 to under 20 kg, fosamprenavir 23 mg/kg plus ritonavir 3 mg/kg; 20 kg or more, fosamprenavir 18 mg/kg plus ritonavir 3 mg/kg. When dosing with ritonavir, do not exceed the adult dose of fosamprenavir 700 mg / ritonavir 100 mg twice daily. Alternatively, PI-naive children aged 2 years and older can be given fosamprenavir without ritonavir 30 mg/kg twice daily. Fosamprenavir should only be given to infants born at 38 weeks' gestation or greater who have attained a postnatal age of 28 days. Treatment is not recommended in PI-experienced children younger than 6 months; once-daily dosing is not supported in any paediatric patient, and twice-daily dosing without ritonavir is not supported below 2 years. Tablets may be used without ritonavir for children weighing at least 47 kg, and with ritonavir for children weighing at least 39 kg (ritonavir capsules from 33 kg). Because the paediatric regimen is a four-band mg/kg table rather than a single clean per-kg dose, no structured paediatric dose is recorded here - verify all paediatric dosing against a children's formulary.

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Previously demonstrated clinically significant hypersensitivity (e.g. Stevens-Johnson syndrome) to any of the components of the product or to amprenavir (US §4)
Co-administration with drugs that are highly dependent on CYP3A4 for clearance and for which elevated plasma concentrations are associated with serious and/or life-threatening events (US §4)
Named contraindicated drugs, applying with or without ritonavir unless otherwise indicated: alfuzosin; flecainide and propafenone (with ritonavir); rifampin; lurasidone (with ritonavir) and pimozide; the ergot derivatives dihydroergotamine, ergonovine, ergotamine and methylergonovine; cisapride; and herbal products - the captured §4 list is truncated at the herbal entry, so the clinician must review the full contraindicated-drug list in the label
If fosamprenavir is co-administered with ritonavir, the full ritonavir prescribing information must also be consulted for additional contraindications

Side effects

Diarrhoea, rash, nausea, vomiting and headache - the most common adverse reactions in adults (incidence 4% or greater) and the most common moderate-to-severe reactions in clinical trials
Severe or life-threatening skin reactions, including Stevens-Johnson syndrome - fosamprenavir should be discontinued for severe skin reactions (US §5.2)
Transaminase elevations - use of higher-than-approved doses may lead to transaminase elevations, and patients with hepatitis B or C are at increased risk (US §5.4)
New onset or exacerbation of diabetes mellitus and hyperglycaemia; elevated triglyceride and cholesterol concentrations (monitor before therapy and periodically thereafter); increase of body fat; immune reconstitution syndrome (US §5.5 to §5.8)
Acute haemolytic anaemia (reported with amprenavir), nephrolithiasis, and spontaneous bleeding in patients with haemophilia for whom additional factor VIII may be required (US §5.9 to §5.11). Vomiting and neutropenia were more frequent in paediatric patients than in adults. Treatment discontinuation due to adverse events occurred in 6.4% of subjects receiving fosamprenavir, most commonly for diarrhoea, nausea, vomiting, raised AST, raised ALT and rash

Interactions

Amprenavir, the active metabolite of fosamprenavir, is an inhibitor of CYP3A4 metabolism and should not be administered concurrently with medications with narrow therapeutic windows that are substrates of CYP3A4 (US §7.1)
Amprenavir also induces CYP3A4 and is itself metabolised by CYP3A4; co-administration with CYP3A4 inducers such as rifampin may decrease amprenavir concentrations and reduce therapeutic effect, potentially leading to loss of virologic activity and resistance (US §7 and §7.1)
Co-administration with drugs that inhibit CYP3A4 may increase amprenavir concentrations and increase the incidence of adverse effects (US §7.1)
Co-administration of fosamprenavir with ritonavir may result in clinically significant interactions with drugs metabolised by CYP2D6, in addition to CYP3A4 (US §7)
Initiation of fosamprenavir/ritonavir (a CYP3A inhibitor) in patients already receiving other medicines, or initiation of other medicines in patients already on fosamprenavir/ritonavir, carries a risk of serious adverse reactions - consult the full prescribing information for potential drug interactions before and during treatment (US §5.1)

Clinical monograph

How it works

It is hydrolysed to amprenavir, which inhibits HIV-1 protease and prevents cleavage of viral polyproteins, producing immature, non-infectious virions.

Prescribing in practice

It is a sulfonamide-derived drug and can cause serious skin reactions, so it should be used with caution in patients with sulfonamide allergy and stopped if a severe rash occurs.
It is extensively involved in cytochrome P450 metabolism and is usually boosted with ritonavir, leading to many clinically important drug interactions.
Like other protease inhibitors it can cause metabolic disturbances including dyslipidaemia and hyperglycaemia.

Monitoring

Monitor HIV viral load and CD4 count, lipids and blood glucose, hepatic function, and for rash or drug interactions.

Counselling the patient

Take with your boosting medicine exactly as prescribed.
Report any rash, particularly if severe or with blistering.
Tell your team about all other medicines, as interactions are common.

Evidence & guidelines

Fosamprenavir is recommended within boosted protease inhibitor regimens in UK (BHIVA) HIV treatment guidance.

Reference: BHIVA; SmPC; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways