Particulates in protein therapeutics, composed of agglomerated protein monomers, may cause an immunogenic response in patients. Consequently, industry and the FDA desire more accurate methods for counting and characterizing particulates. Unlike likely manufacturing impurities, protein particulates have a refractive index very close to that of the matrix solution. In this paper, we describe some of the measurement challenges for the common optical methods of brightfield microscopy and light obscuration. At NIST, we are developing two types of reference materials that mimic the properties of protein particulates to support the validation of optical particle counters. We have developed a polymer-based protein surrogate based on the partially fluorinated polymer ethylene tetrafluoroethylene (ETFE). Mechanical abrasion of the ETFE followed by size filtration produces a polydisperse suspension of very rugged particles with a morphology quite similar to protein particulates. In another approach, we have created artificial particles polymer particles as small as 3 µm x 4 µm x 40 µm in large quantities using photolithographic methods developed in the semiconductor industry. Future work will focus on reducing the optical contrast of these particles. These reference materials will provide a stringent test of the sensitivity and morphological measurements of optical particle-counting instruments.
Citation: American Pharmaceutical Review
Pub Type: Journals
agglomerate, aggregate, light obscuration, microscopy, particle counter, particulate, protein