Biogen said on Tuesday its second-quarter earnings topped expectations on higher sales of its drugs to treat multiple sclerosis, and the company increased its profit forecast for the year.

The company's shares rose 3.5% in premarket trading.

Higher-than-expected sales of Avonex, its biggest-selling multiple sclerosis drug and Rituxan, its cancer drug, offset lower-than-expected sales of its newer multiple sclerosis drug Tysabri.

The Weston, Massachusetts-based biotechnology company raised its full-year profit forecast to above $6.20 a share from a previous forecast of above $6.15 a share. It left its revenue guidance unchanged for growth in the mid-single digits.

Mark Schoenebaum, an analyst at ISI Group, said the forecast could be conservative.

Net profit rose to $387 million, or $1.61 a share, from $288 million, or $1.18, a year earlier. Last year, the company's earnings were hurt by a charge of 15 cents per share due to arbitration involving its partner Roche Holding.

Excluding one-time items, the company earned $1.82 a share. Analysts were on average expecting earnings of $1.56 a share, according to Thomson Reuters I/B/E/S.

Revenue rose 18% to $1.4 billion, driven by sales of the Avonex pen, a new form of delivery of the drug. Avonex sales rose 16% to $762 million.

Analysts were expecting Avonex sales of about $698 million and total revenue of $1.33 billion.

Global sales of Tysabri, which is administered once a month by infusion and is the company's biggest growth driver, rose 2% to $395 million. Analysts were expecting sales of $419 million.

Revenue from Rituxan was $285 million. Analysts were expecting revenue of $266 million. The company sells the cancer drug as part of a joint venture with Roche.

New products

Biogen's shares have risen 70% over the past year, largely on optimism about its experimental new multiple sclerosis drug, BG-12, which is expected to be approved as early as the end of this year.

BG-12 would be the second oral drug to reach the market and would compete with Gilenya, an oral treatment from Novartis. Sales of Gilenya rose about 15% in the second quarter to $283 million.

That level of growth was stronger than some investors had expected. In April, European and U.S. regulators ruled that the drug should carry stronger warnings about its potential to cause heart problems.

Novartis said it expects sales of Gilenya to exceed $1 billion this year.

Analysts expect sales of BG-12 of $392 million in 2013 and $1.02 billion in 2014, according to Brian Abrahams, an analyst at Wells Fargo Securities.

BG-12, which appears to have a more benign safety profile than Gilenya, is ultimately expected, if approved, to become the leading MS drug on the market.

In addition to hopes for BG-12, Biogen's shares have been lifted by improved sentiment toward Tysabri, which was temporarily withdrawn from the market in 2005 after being linked with a potentially deadly brain infection known as PML. It was reintroduced in 2006 with stricter warnings but safety concerns have remained ever since.

Physicians are increasingly adopting the use of a test that screens patients for the JC virus, which is thought to cause PML.

Investors are also awaiting the release later this year of data from late-stage clinical trials of Biogen's experimental treatments for hemophilia and amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease.