BENGALURU, Dec 29 (Reuters) - Indian shares edged lower in thin year-end trading on Wednesday as weaker metal stocks offset a rally in pharmaceutical firms ahead of the expiration of monthly derivatives. Snapping two sessions of gains, the NSE Nifty 50 index (.NSEI) closed down 0.1% at 17,213.60 and the benchmark S&P BSE Sensex (.BSESN) fell 0.16% to 57,806.49. "We are nearing expiry (of derivatives) and it"s December-end ... there is volatility and we are also coming close to some amount of stability in the markets," said Anita Gandhi, a whole-time director at Arihant Capital Markets. Broader Asian shares also ticked lower, following a mixed Wall Street session, as the region"s investors positioned their portfolios for the new year. Indian equities have retreated more than 7% from a record peak hit in October on worries over high valuations and the spread of the Omicron variant of COVID-19 across the globe. "Overall volumes have fallen and markets are consolidating in thin volumes. A mild rally over the year-end also cannot be ruled out because of net asset value-based buying," Gandhi said. The Nifty pharma index (.NIPHARM) was the top gainer, rising 1.7% a day after India approved Merck"s (MRK.N) COVID-19 pill and two more vaccines for emergency use. read more Earlier this year, several drugmakers including Dr Reddy"s Labs (REDY.NS), Aurobindo Pharma (ARBN.NS), Cipla (CIPL.NS) and Sun Pharmaceuticals (SUN.NS) had signed deals with Merck to manufacture and supply its molnupiravir pill in India. Eicher Motors (EICH.NS) and Sun Pharma were the top gainers on the Nifty 50 index, rising 3.30% and 2.9%, respectively. Metal stocks were the top drags, with the nifty metals index (.NIFTYMET) falling 1.1%. India"s market regulator on Tuesday strengthened rules for companies going public, potentially slowing some planned new issues, as it seeks to protect retail investors after a record year of initial public offerings. read more