AstraZeneca has suffered another clinical setback after its investigational rheumatoid arthritis pill fostamatinib failed to match the efficiency of Abbott's blockbuster Humira.

The Anglo-Swedish drugmaker has announced top-line results of a Phase IIb monotherapy study of fostamatinib, which is the first kinase inhibitor with selectivity for SYK (spleen tyrosine kinase) in development as an oral treatment for RA. In the trial, fostamatinib met its first primary objective, showing statistically significant superiority compared to placebo at six weeks at the 100mg twice-daily dose and the 100mg twice-daily for a month followed by 150mg once-daily dose, but not at the 100mg twice-daily for a month followed by 100mg once-daily dose.

However, observers have focused on the second endpoint whereby all fostamatinib monotherapy doses were inferior to monotherapy with Humira (adalimumab), which is injectable. Martin Mackay, R&D president at AstraZeneca, said the dose-finding study was designed to evaluate the effect of fostamatinib independent of methotrexate "and to inform the further development of fostamatinib as a monotherapy treatment for RA".

He added that a more comprehensive assessment of the benefit/risk profile of fostamatinib used in combination with a disease-modifying antirheumatic drug is being undertaken in the studies that form the Phase III programme which are on track to report in the first half of 2013". Dr Mackay said that regulatory filings in the USA and the European Union for the drug, which is licensed from Rigel Pharmaceuticals, are expected in the second half of next year.

Still the results have not gone down well with investors and AstraZeneca shares had slipped 2.3% at 10.30 am (UK time) to £29.72. In an investor note, Savvas Neophytou at Panmure Gordon, said that the results "confirm the efficacy signal…but failure to show it can match Humira's efficacy are likely to limit the product's commercial potential".

Nonetheless, he added that these studies "are designed to provide direction for the Phase III programme and to that end we are making no changes to forecasts".