(4f) Polymeric Biomaterials with Tailored Microstructures, Nanostructures, and Bioactive Surface Chemistries for Drug Delivery and Tissue Engineering

Matt J. Kipper

Introduction

My research involves biomedical applications of chemical engineering, by applying polymer
science to relevant problems in drug delivery and tissue engineering.

I earned my
Ph.D. from Iowa State University, where my work resulted in the development of
a single-dose vaccine delivery vehicle based on bioerodible polymer
microspheres with tailored microstructures and nanostructures. This work
involved:

• Experimental and theoretical
  investigation of the polymer physics of the particular polymer system in order
  to determine its microstructure and nanostructure.

• Mathematical modeling of the erosion
  and drug release kinetics.

• Studies to investigate the vaccine
  efficacy and capability to modulate the immune response mechanism.

This work
resulted in the publication of twelve peer-reviewed journal articles and one
patent.

Currently,
I have a postdoctoral fellowship award from the National Research Council with
a joint appointment at the National Institutes of Health and the National
Institute of Standards and Technology. I am developing surfaces and scaffolds
that present covalently attached bioactive chemistries that promote the
migration of specific cell types important to tissue engineering. By promoting
migration, these materials could overcome challenges associated with cell
seeding on three-dimensional scaffolds and enhance wound healing. Additionally,
I am demonstrating how these materials can aid cellular and developmental biologists
in studying phenomena that contribute to cell migration. In addition to tissue
engineering, this work could have impacts in areas such as embryonic
development and tumor metastasis, for which cell migration is a critical
phenomenon. I'll be presenting this work at talk 594f on Friday, November 4^th
at 1:34 PM in Regency Ballroom G.

These two
projects are briefly outlined below:

Single-dose
vaccine based on bioerodible microspheres with tailored microstructures and
nanostructures

In 2003,
the National Institutes of Health, the World Health Organization, and the Bill
and Melinda Gates Foundation identified the development of single-dose vaccines
as number one on the list of Grand Challenge in Global Health (http://www.grandchallengesgh.org/).
This work describes the investigation of bioerodible polyanhydrides as
controlled drug and vaccine delivery vehicles, and the development of a
single-dose vaccine carrier based on these materials. The polymers studied are
based on the 1,6-bis(p-carboxyphenoxy)hexane (CPH) and sebacic acid (SA)
monomers. These two materials erode at vastly different rates and can be
combined in random copolymers or blends to achieve tailored erosion kinetics.
The hydrophobic nature of these materials offers the potential to stabilize
proteins, and their mutual incompatibility and semicrystallinity provide an
interesting phase behavior, which can be exploited to aid in tailoring the
release kinetics. This work involved the theoretical and experimental
description of the microstructure and nanostructure of polyanhydride
copolymers, the development of an injectable drug delivery vehicle based on
polyanhydride microspheres, and the development of accurate kinetic models of
polymer erosion that provide details of the erosion phenomenon that are
difficult to obtain experimentally, but may impact the stability of therapeutic
proteins.

Description
of the microstructure and nanostructure of polyanhydride copolymers

Through a
combination of small angle X-ray scattering, atomic force microscopy,
solid-state NMR, optical microscopy, and molecular simulations, detailed
descriptions of the copolymer and blend microstructures and nanostructures are
obtained. This microstructure/nanostructure includes microphase separation in
the amorphous phase for the copolymers, crystalline/amorphous phase separation
for the copolymers and homopolymers, and a phase diagram for the hompolymer
blends. This comprehensive description of the microstructural details is essential
to understanding the complex erosion and drug release kinetics exhibited by
these materials.

Development
of an injectable drug delivery system based on polyanhydride microspheres

Release
kinetics experiments are performed in vitro and in vivo to ascertain
the affects of microstructural and nanostructural characteristics and to study
the immune responses to a model antigen, tetanus toxoid (TT). Tailored release
profiles of small molecular weight model drugs are demonstrated by combining
microspheres with different erosion kinetics in ?cocktails.? Several vaccine
formulations are investigated to determine which combinations of polymer
hydrophobicity, protein stability, and protein release kinetics offer the
greatest potential to achieve protective immunity in a single dose. A
single-dose vaccine formulation that induces a secondary immune response
characterized by sustained high titers of high avidity antibody is demonstrated
in a mouse model. Additionally, it is shown that the in vivo immune
response mechanism can be tuned by altering the vaccine formulation. The
ability to alter the immune response mechanism without the addition of noxious
adjuvants is a unique and valuable feature of this delivery system.

Accurate
erosion and drug release kinetics models

The models
incorporate the details of the polymer microstructure and provide molecular
level descriptions of the complex process of erosion. Important phenomena that
occur during erosion, such as porosity change, crystallinity change, monomer
accumulation, and pH change in the eroding zone are described. Although these
phenomena are difficult to accurately measure experimentally, they can be
predicted by accurate erosion models such as the one presented. Understanding
these phenomena is essential to the rational design of controlled release
systems for macromolecular drugs (e.g. proteins, vaccines).

Bioactive
peptide gradients for promotion and assay of cell migration

The
migration of cells that are involved in the early (inflammation) stage of wound
healing, such as neutrophils, macrophages, and T lymphocytes has been
extensively studied. However, the migration of slower migrating cells, such as
fibroblasts and endothelial cells, which are involved in the later stages
(proliferation and remodeling) of wound healing, is more difficult to study.
Techniques such as Boyden chamber assays, in which the cells are allowed to
migrate across a membrane or filter may be good models for processes such as
metastasis or extravasation, but may not be relevant models for connective
tissue cells migrating across a wound. Cells migrate to a wound in response to
concentration gradients of soluble chemotactic factors by a process known as
chemotaxis. Because these gradients are inherently unstable, it is difficult to
use them to study the migration of slower moving endothelial cells and
fibroblasts. Endothelial cells and fibroblasts can also respond to bound
(rather than soluble) peptide gradients in a process known as haptotaxis. This
process may be exploited in novel tissue engineering scaffolds to recruit cells
to a wound site and promote the migration of cells into a tissue engineering
construct. A technique for preparing surfaces and three-dimensional scaffolds
with covalently bound peptide gradients is discussed. Bioactive peptides from
Laminin-1, a basement membrane protein, that are known to promote adhesion or
migration of endothelial cells or fibroblasts are used to develop surfaces and
three-dimensional scaffolds with covalent peptide gradients. Time-lapse video microscopy
of cell culture is used to monitor the behavior of cells with respect to the
gradients. This approach offers a new technique for screening the haptotactic
potential of peptides. It also permits the study of haptotaxis for slowly
migrating cells that are difficult to characterize by other techniques.
Finally, these materials are readily adaptable to clinical applications of
tissue engineering as they do not contain unstable gradients and are based on
materials with well-established biocompatibility for a variety of in vivo
applications.

Journal
Publications

1. A.S. Determan, J. Wilson, M.J. Kipper,
M. Wannemuehler, and B. Narasimhan, ?Protein Stability in the Presence of
Polymer Degradation Products: Consequences for Controlled Release Formulations.?
Biomaterials, (Submitted, 2005).

2. M.J. Kipper, J. Wilson, M. Wannemuehler, and B. Narasimhan,
?Single dose Tetanus Vaccine based on Bioerodible Polyanhydride Microspheres
can Modulate Immune Response Mechanism.? J. Biomed. Mater. Res. A., (In
press, 2005).

3. M.J. Kipper, S. Seifert, S.-S. Hou, K. Schmidt-Rohr, P.
Thiyagarajan, and B. Narasimhan, ?Nanoscale Morphology of Polyanhydride
Copolymers.? Macromolecules, 38, 8468-8472, 2005.

4. M.J. Kipper, S. Seifert, P. Thiyagarajan, and B. Narasimhan,
?Small-Angle X-Ray Scattering to Discern Microstructure of Semicrystalline
Polyanhydrides for Drug Delivery.? Adv. X-Ray Anal., 48, 73-81,
2005.

5. M.J. Kipper, and B. Narasimhan, ?A Molecular Description of
Erosion Phenomena in Biodegradable Polymers.? Macromolecules, 38,
1989-1999, 2005.

6. M.J. Kipper, S. Seifert, P. Thiyagarajan, and B. Narasimhan,
?Morphology of Polyanhydride Copolymers:  Time Resolved SAXS Studies of
Polyanhydride Crystallization.? J. Polym. Sci., Part B: Polym. Phys., 43,
463-477, 2005.

7. M.J. Kipper, S. Seifert, P. Thiyagarajan, and B. Narasimhan,
?Understanding Polyanhydride Blend Phase Behavior Using Scattering, Microscopy,
and Molecular Simulations.? Polymer, 45, 3329-3340, 2004.

8. B. Narasimhan and M.J. Kipper,
?Surface-Erodible Biomaterials for Drug Delivery.? Adv. Chem. Engr., 29,
169-218, 2004.

9. C. Berkland, M.J. Kipper, K. Kim,
B. Narasimhan, and D.W. Pack, ?Microsphere Size, Precipitation Kinetics and
Drug Distribution Control Drug Release from Biodegradable Polyanhydride
Microspheres.? J. Controlled Release, 94, 129-141, 2004.

10. M. J. Kipper, E. Shen, A. Determan, and B. Narasimhan, ?Design of
an Injectable System Based on Bioerodible Polyanhydride Microspheres for
Sustained Drug Delivery.? Biomaterials, 23, 4405-4412, 2002.

11. D. Larobina, M.J. Kipper, G.
Mensitieri, and B. Narasimhan, ?Mechanistic Understanding of Degradation in
Bioerodible Polymers for Drug Delivery.? AIChE J., 48, 1260-1270,
2002.

12. E. Shen, M.J. Kipper, B. Dziadul,
M.-K. Lim, and B. Narasimhan, ?Mechanistic Relationships Between Polymer
Microstructure and Drug Release Kinetics in Bioerodible Polyanhydrides.? J.
Controlled Release, 82, 115-125, 2002.

Patent

M.J. Kipper, B. Narasimhan, J.H. Wilson, and M.J. Wannemuehler,
Single-Dose Controlled-Release Vaccine Formulations Based on Polyanhydride
Microspheres for Control of Immune Response Mechanism, U.S. patent application
number 60/623,711, October 29, 2004.