filtration sterilization
Some drugs would be destroyed by the heat of an autoclave or oven. Filtration sterilization gets around this by physically straining the microbes out instead of killing them: the liquid is forced through a membrane whose pores are too small for bacteria to pass, so what comes out the other side is sterile while the drug never gets hot.
The standard sterilizing-grade filter has a nominal pore size of 0.22 micrometres (sometimes written 0.2), small enough to retain bacteria. The mechanism is partly simple sieving — particles bigger than the pore are blocked — and partly adsorption of smaller organisms onto the membrane. Because the product itself is never heated, this is the method of choice for heat-labile solutions: protein and peptide drugs, certain antibiotics, vitamins and biologicals.
Filtration is not a terminal method. The sterile filtrate must be collected and filled into its container under aseptic conditions, because the filter cleans the liquid but cannot keep it clean afterwards. The integrity of the filter is verified before and after use by a bubble-point or pressure-hold test to prove no microbe-sized leak existed.
Two honest caveats. A standard 0.22 micrometre filter retains bacteria and fungi but does not remove viruses or dissolved endotoxin, which are smaller; and a damaged or oversized-pore filter could pass organisms undetected, which is why integrity testing is mandatory rather than optional.
A monoclonal antibody solution that would denature if heated is passed through a 0.22 micrometre filter into a sterile holding tank, then aseptically filled into vials.
Strain the microbes out instead of cooking them — the answer for heat-labile drugs.
Filtration sterilization only makes sense paired with aseptic processing. The filter delivers a sterile liquid, but everything downstream — the receiving vessel, the filling line and the closures — must already be sterile and kept that way.