High resolution removal of virus from protein solutions using a membrane of unique structure.

We describe a new class of membrane that has the capability of removing particles such as viruses from solution with resolution and reproducibility superior to that of conventional membranes. This composite membrane is composed of a pre-formed microporous membrane plus a thin asymmetric, finely porous retentive layer that is quite different from conventional ultrafilters. The protein sieving characteristics of this membrane are nearly equivalent to, but slightly less than, that of conventional 100,000 Dalton cut-off ultrafiltration membranes. This membrane uniquely shows particle retention characteristics that increase monotonically from 3 to 8 logs as a function of particle diameter in the range of 28 to 93 nm. The performance of this membrane in both a single stage and a two stage system show that 4 to 6 log overall removal of virus particles in the size range 30 to 70 nm is possible with simultaneous high recovery of product protein. Clearance factors exceeding 6 logs are possible with viruses larger than 78 nm. In addition, the performance of process systems containing this membrane is predictable in accordance with the general membrane properties and equilibrium mass balance models. This membrane system is fully validatable and can be used in conjunction with other validated operations in a down-stream process to reliably achieve an over-all reduction of 12 logs of known or putative virus particles.