The pharmaceuticals and biotechnology sector is the weakest sector Friday afternoon with AstraZeneca among the biggest fallers.AstraZeneca has tumbled after it suffered another delay in getting US approval for its potential blockbuster blood-thinning drug Brilinta after the US Food and Drug Administration requested more information. The FDA issued AstraZeneca with a complete response letter requesting additional analysis of the data from a study of the effects of Brilinta (ticagrelor) on 18,600 patients in 43 countries compared with its main rival Plavix.The banking sector continues to fall after gloomy comments by the Bank of England about potential bad debt provisions, with RBS, Lloyds and Barclays all posting losses.Additionally ratings agency Moody's slashed its credit rating on Ireland by five notches to Baa1 from Aa2 amid worries over banking liabilities and economic prospects. Bank of Ireland leads the sector lower.The alternative energy sector is the best performing sector led by US based bio-refining company GTL Resources and solar power technology group Jetion Solar Holdings.Top performing sectors so far todayAlternative Energy 4,027.81 +3.42%Forestry & Paper 5,235.96 +2.19%Industrial Metals & Mining 7,841.96 +1.47%Oil Equipment, Services & Distribution 24,223.55 +1.40%Mobile Telecommunications 3,890.70 +1.20%Bottom performing sectors so far todayPharmaceuticals & Biotechnology 8,984.26 -2.05%Banks 4,750.00 -1.62%Aerospace and Defence 3,294.16 -0.89%Household Goods & Home Construction 6,603.89 -0.72%Life Insurance 3,981.62 -0.67%