Amgen’s 20% jump in fourth quarter earnings was not enough to avoid a drop in the firm’s share price yesterday, as investors grew skittish about the firm’s decision to delay reporting data on its new cancer drug panitumumab and the potential threat of ‘generic’ competition to its anaemia drugs.

The world’s biggest biotechnology company’s shares fell nearly 5% yesterday to close at $71.90, despite posting net earnings of $824 million on revenues that swelled 12% to $3.3 billion, as anaemia treatment Aranesp (darbepoetin alfa) performed better than expected.

There had been concerns that changes to the Medicare reimbursement system would hold back sales of Aranesp, but it shrugged these off with a 24% increase to $873 million. Fellow anaemia drug Epogen (epoetin alfa) suffered for the success of its counterpart, however, dipping 10% to $626 million due to cannibalisation by Aranesp, as well as lower demand.

George Morrow, Amgen’s executive vice president, said conversions to Aranesp are expected to stabilise by the middle of this year, allowing ‘modest’ growth to resume for Epogen thereafter.

Epogen’s decline was also counterbalanced by good gains for arthritis and psoriasis drug Enbrel (etanercept), which rose 19% to $674 million in the quarter, as well as the white cell stimulators Neupogen (filgrastim) and Neulasta (pegfilgrastim), up 22% to $729 million. Neulasta in particular was helped by new labelling extending its use into first-cycle chemotherapy regimens.

For the full year, sales came in at $12.4 billion, up 18%, while net income rose 28% to $4 billion. Amgen said it expected 2006 revenues to be in the range of $13.9 to $14.4 billion, while R&D expenses will rise sharply as it funds several late-stage projects, including the much-touted osteoporosis drug denosumab.

But with these two anaemia products still accounting for 47% of Amgen’s total sales in the quarter, the prospect of 'biosimilar' versions of erythropoietin, the growth factor on which they are based, weighs heavy on investor’s minds.

Generics company Stada said recently it expects to file for approval of a ‘biosimilar’ version of EPO in the European Union in the first-quarter of 2006, while Sandoz and Pliva are also developing versions for the EU market. Pliva’s product has already been launched in its home market of Croatia, and there is every chance that the first copycat rivals will appear on the market this year.

Amgen is hoping that Aranesp, which requires fewer injections and has been updated to use more patient-friendly pen injector devices, will provide the franchise with some protection from the biosimilars.

But investors seemed particularly worried about a decision to delay reporting interim clinical data on panitumumab, a drug for colorectal cancer that will compete in the marketplace with ImClone’s Erbitux (cetuximab), sold by Bristol-Myers Squibb and Merck KGaA. This has been tipped as a $2 billion product by the company, which said the delay was merely a result of slower-than-expected patient recruitment into the studies.