* Two patents relating to dosage and composition revoked
* Hospira says move paves way for its biosimilar version
* Roche considering its next steps (Adds Roche and Hospira comment, patent details)
LONDON, April 10 (Reuters) - Hospira has successfully overturned two patents on Roche's blockbuster breast cancer drug Herceptin in Britain, clearing the way for it to launch a cheaper copycat version in the country.
A ruling handed down by the High Court in London on Thursday determined that both the 115 and 455 patents on the drug, which relate to the drug's dosage and its composition, were invalid.
The basic underlying patent held by Roche on its medicine was not challenged by Hospira. This patent expires on July 28 and Hospira told the court it wanted to sell its version of the product, known generically as trastuzumab, after that date.
Such a move would ramp up competition for a costly cancer treatment and could drive down prices. The potential to use copycat forms of biotech drugs like Herceptin is gaining increased attention from cost-conscious health authorities across Europe.
"We are very pleased with this decision, which helps pave the way for our trastuzumab product," U.S.-based Hospira said in a statement.
A spokeswoman for Roche said the Swiss drugmaker was analysing the court ruling revoking its patents and considering its next steps.
Herceptin, an injectable antibody-based biotech drug, had worldwide sales of 6.08 billion Swiss francs ($6.90 billion) last year, making it the company's third biggest-selling medicine after two other cancer drugs, MabThera and Avastin. British sales are a small proportion of that total.
Herceptin is designed to fight cancers that have too much of a protein called human epidermal growth factor receptor 2, or HER2, on the surface of their cells. That means it is only helpful for a minority of women with breast cancer but for this group it has proved to be an important treatment.
Hospira has pioneered the development of so-called biosimilar versions of biotech drugs. Unlike conventional chemical medicines, these biological drugs are complex to make and copies can only ever be similar to the original product rather than exact replicas.
Hospira recently launched the first antibody biosimilar in Europe, called Inflectra, which is a copy of Johnson & Johnson's and Merck & Co's Remicade drug for rheumatoid arthritis.
($1 = 0.8807 Swiss Francs) (Additional reporting by Kate Holton, Editing by Jane Merriman)