Gilead Sciences saw a healthy hike in fourth-quarter revenues driven by demand for its combination HIV drug Truvada and accelerating royalties from influenza drug Tamiflu.

The infectious disease specialist said sales came in at $609 million, a rise of 65% over the same period of 2004, helped by an 18% advance for Truvada (emtricitabine and tenofovir disoproxil fumarate) to $191 million. Truvada was launched in the USA in 2004 and won European approval in February 2005.

Overall, Gilead’s core HIV franchise posted a 47% sales increase to just under $385 million, although its single-agent products Viread (tenofovir) and Emtriva (emtricitabine) were both victims of Truvada’s success, dropping 8% and 15%, respectively, to $182 million and $11 million.

Gilead’s other growth drivers were chronic hepatitis B treatment Hepsera (adefovir dipivoxil), up 43% to $51 million, as well as royalties on burgeoning sales of Roche’s flu drug Tamiflu (oseltamivir phosphate), which added $101 million to the pot in the fourth quarter, and $161 million for the full year, as sales of the product surged amid fears of a flu pandemic.

The company said it expects 2006 sales of HIV drugs to increase by 20%-25% over 2005 levels, with the gains coming in part from Truvada’s roll-out in additional markets.

Meanwhile, there is a chance that another HIV product, based on Truvada and Bristol-Myers Squibb’s Sustiva (efavirenz), could reach the market late this year and lend some additional momentum to Gilead’s HIV franchise – although this assumes that the two companies can iron out some bioequivalency issues that have emerged in clinical trials.

Gilead has said it hopes to file for this product in the second quarter of this year.