Parenteral Delivery of Peptides and Proteins - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 553

batch uniformity, and interbatch reproducibility, adjustable release profiles, low burst
effect, syringeability, and free-flowing microspheres. Figure 11.6 demonstrates the
release mechanism of P/P drugs from microspheres.
PLGA is the most commonly used polymer for encapsulating many P/P drugs like
CyA [195] , neurotensin [196] , and salmon calciton [151] . PEG-PLGA degradable
copolymeric microspheres containing BSA have been used to maintain high systemic
levels of this hormone [197] . Gelatin is also used for preparation of microspheres to
deliver proteins and peptides [198] .
11.5.2 Implants
Implantable polymeric depots for protein and peptide drugs have a number of potential
advantages such as site-specific delivery that is effective at a low concentration over
a prolonged period. Implants are often placed subcutaneously by a large-bore needle,
pellet injectors, or minor surgery. In implants, the protein drug is distributed through-
out the polymeric matrix. The release of the protein from the implant is governed
by three phases: (1) initial burst, (2) diffusion-controlled release, and (3) erosion-
controlled release. Microspheres and cylinders are the most favorable designs used
in marketed products. The controlled implantable delivery system avoids the highly
variable peak and through drug concentration often seen with immediate release
delivery systems (injections, tablets).
Figure 11.6 Various mechanisms of P/P drug release from microspheres. (A) Pulsatile
release, (B) burst release, and (C) continuous release.