FIELD OF THE INVENTION The present invention relates to novel pharmaceutical sustained release compositions and process of preparation of such compositions preferably comprising active agent(s) having good bioavailability. Particularly this invention pertains to pharmaceutical compositions comprising antiviral active agent, process of preparation of such compositions and method of using them. BACKGROUND OF THE INVENTION The advantages of sustained release products are well known in the pharmaceutical field and include the ability to slowly release the medicament over a period of time while increasing patient compliance by reducing the number of administrations necessary to achieve the same level. Various attempts to provide dosage forms for delivery of active agent that remain in the stomach for extended periqd,s. pr time, have been described previously.