Engineering Antibody Fabs for Long Acting Delivery to the Eye

Maximal clinical benefit for treatment of back of the eye disorders such as agerelated
macular degeneration (AMD) with protein therapeutics is currently obtained
with intravitreal injections at 4-8 week intervals. Use of long acting delivery (LAD)
technologies or molecules with increased vitreal half-life would benefit patients
through reduced dosing frequency, less incidence of injection-related complications,
and perhaps improved durability of treatment outcome. Challenges for LAD to the
eye or clearance modification include restrictions on volume (≤100 μL) of injection
necessitating high concentration formulations, a short list of formulation excipients
approved for ocular use, stability of proteins to vitreous humor conditions, and a
limited understanding of the mechanism of protein clearance from the eye. We have
been using pharmacokinetic (PK) studies in a rabbit model to explore molecular
attributes of antibody Fab fragments affecting vitreal half-life. We find that
hydrodynamic size is a key contributor to the rate of vitreal clearance, suggesting
that diffusion out of the vitreous cavity is a rate-determining step, with vitreal halflife
showing a linear dependence on hydrodynamic radius calculated from light
scattering measurements. Increasing the hydrodynamic size of a protein
therapeutic slows clearance and is an effective means to achieve less frequent
dosing. In addition, we present a case study involving the re-engineering of a
therapeutic Fab to increase the stability to vitreous humor conditions yielding
variants that are more suitable for LAD.