Dr. Burgess’s research efforts focus on drug delivery systems: microspheres, emulsions, liposomes, hydrogels, as well as interfacial chemistry and span the following areas: development of novel technologies; preformulation; materials science, formulation; stability assessment and prediction; transport; sterility assessment; biophysical assessment; cell culture and development of in vitro performance tests and in vitro-in vivo correlation. She is currently developing a miniaturized, implantable biosensor for metabolic monitoring, investigating and controlling tissue response to the implant. She is also investigating intracellular trafficking of DNA therapeutics.
Dr. Burgess research has attracted funding from government agencies (specifically the NIH, the United States Army, Telemedicine and Advanced Technology Research Center (TATRC), and Food and Drug Administration (FDA) ), pharmaceutical and related industries as well as research foundations such as Juvenile Diabetes Association (JDA) and the Center for Pharmaceutical Processing Research (CPPR).
TOTALLY IMPLANTABLE WIRELESS GLUCOSE SENSORS
Real time monitoring of various metabolic analytes that control function and physiology of the human body is crucially needed for a variety of applications and especially for diabetic patients in everyday life. Our group, in collaboration with the groups of Professors Papadimitrakopoulos and Jain, has been developing wireless, totally implantable glucose sensors that exhibit significant size reduction, increased bio-acceptability and suppression of inflammation (J. Contr. Release, 2007, 117, 68). Significant effort is exerted to identify the various failure mechanisms and improve upon both in vitro and in vivo device stability (J. Diabetes Sci. & Tech., 2007, 1(2), 193).
LABELING AND INTRACELLULAR TRACKING OF FUNCTIONALLY ACTIVE PLASMID DNA WITH SEMICONDUCTOR QUANTUM DOTS
Semiconductor nanocrystal quantum dots (QDs) allow long-term imaging in the cellular environ- ment with high photostability. QD biolabeling techniques have previously been developed for tagging proteins and peptides as well as oligonucleotides. In this contribution, QD-decorated plasmid DNA was utilized for the first time for long-term intracellular and intranuclear tracking studies. Conjugation of plasmid DNA with phospholipid-coated QDs was accomplished using a peptide nucleic acid (PNA)–N-succinimidyl-3-(2-pyridylthio) propionate linker. Gel electrophoresis and confocal and atomic force microscopy (AFM) were used to confirm the structure of QD–DNA conjugates. AFM imaging also revealed that multiple QDs were attached in a cluster at the PNA- reactive site of the plasmid DNA. These QD–DNA conjugates were capable of expressing the reporter protein, enhanced green fluorescent protein, following transfection in Chinese hamster ovary (CHO-K1) cells with an efficiency of ca. 62%, which was comparable to the control (unconjugated) plasmid DNA. (C. Srinivasan, J. Lee, et al. Molecular Therapy, 2006, v14,issue 2, p192-201) (Link)
SCANNING PROBE MICROSCOPY METHOD FOR NANOSUSPENSION STABILIZER SELECTION
An atomic force microscopy (AFM) method was successfully developed and utilized for investigating the interaction of polymeric stabilizers with ibuprofen to determine their suitability for the preparation and stabilization of ibuprofen nanosuspensions. Images obtained clearly showed that HPMC and HPC interacted strongly with the ibuprofen resulting in extensive surface adsorption, confirming their suitability for ibuprofen nanosuspension preparation. In addition, differences in the morphology of the adsorbed HPMC and HPC molecules were observed, which may be attributed to their variable degree of substitution. Consistent with their poor performance in stabilizing the ibuprofen nanosuspensions, images obtained with PVP and Poloxamer’s depicted inadequate adsorption on the ibuprofen surface. Careful analysis of the AFM images and the ibuprofen crystal structure gave valuable insight into the success of top-down processing for the preparation of nanosuspensions as compared to bottom-up processing. On the basis of the relationship observed between nanosuspension stability and adsorption characteristics of specific polymers, such AFM studies can aid in the selection of suitable nanosuspension stabilizers. This method provides the basis for a scientific rationale for nanosuspension stabilizer selection rather than the trial and error method which is currently practiced. (Link)
A NOVEL USP APPARATUS 4 BASED RELEASE TESTING METHOD FOR DISPERSED SYSTEMS
A novel dialysis adapter has been developed for USP apparatus 4 for in vitro release testing of dispersed system dosage forms. This USP apparatus 4 method was optimized and compared with currently used dialysis and reverse dialysis sac methods. Optimization studies for the USP apparatus 4 method showed that release from solution, suspension and liposome formulations was not flow rate limited and was not affected by change in the dialysis adapter sample volume from 250microl to 500microl. The USP apparatus 4 method could discriminate between solution, suspension and liposome formulations of dexamethasone. On comparing the different methods, only the USP apparatus 4 method provided discrimination between dexamethasone release from extruded and non-extruded liposomes, as well as among non-extruded DMPC, DPPC and DSPC liposomes. The dialysis sac method could not discriminate between the release profiles of non-extruded DMPC and DPPC liposomes. The reverse dialysis sac could not discriminate between the release profiles of extruded and non-extruded DMPC liposomes. In addition, the USP apparatus 4 method provided the highest release and the smallest variation in the data. This novel adapter might address the problem of the lack of a compendial apparatus for in vitro release testing of dispersed system dosage forms. (Link)
PLGA/PVA HYDROGEL COMPOSITES FOR LONG-TERM INFLAMMATION CONTROL FOLLOWING S.C. IMPLANTATION
Dexamethasone loaded PLGA microsphere/PVA hydrogel composites were investigated as an outer drug-eluting coating for implantable devices to provide protection against the foreign body response. Two populations of microspheres were prepared: 25 kDa PLGA microspheres which had a typical triphasic release profile extending over 30–33 days; and 75 kDa PLGA microspheres which showed minimal release for the first 25 days and then increased to release over 80–85 days. Incorporation of the microspheres in the composites only slightly altered the release profile. Composites containing 25 kDa microspheres released dexamethasone over 30–35 days while composites containing combinations of 25 and 75 kDa microspheres in equal amounts released over 90–95 days. Pharmacodynamic studies showed that composites containing only 25 kDa microspheres provided protection against the inflammatory response for 1 month, however, a delayed tissue reaction developed after exhaustion of dexamethasone. This demonstrated that sustained release of the anti-inflammatory agent is required over the entire implant lifetime to control inflammation and prevent fibrosis. Composites fabricated using combinations of 25 kDa and 75 kDa microspheres controlled the tissue reaction for 90 days. This strategy of combining different microsphere populations in the same composite coating can be used to tune the release profiles for the desired extent and duration of release. Such composites offer an innovative solution to control the foreign body response at the tissue–device interface. (Link)
NFLUENCE OF PHASE INVERSION ON THE FORMATION AND STABILITY OF ONE-STEP MULTIPLE EMULSIONS
A novel method of preparation of water-in-oil-in-micelle-containing water (W/O/Wm) multiple emulsions using the one-step emulsification method is reported. These multiple emulsions were normal (not temporary) and stable over a 60 day test period. Previously, reported multiple emulsion by the one-step method were abnormal systems that formed at the inversion point of simple emulsion (where there is an incompatibility in the Ostwald and Bancroft theories, and typically these are O/W/O systems). Pseudoternary phase diagrams and bidimensional process-composition (phase inversion) maps were constructed to assist in process and composition optimization. The surfactants used were PEG40 hydrogenated castor oil and sorbitan oleate, and mineral and vegetables oils were investigated. Physicochemical characterization studies showed experimentally, for the first time, the significance of the ultralow surface tension point on multiple emulsion formation by one-step via phase inversion processes. Although the significance of ultralow surface tension has been speculated previously, to the best of our knowledge, this is the first experimental confirmation. The multiple emulsion system reported here was dependent not only upon the emulsification temperature, but also upon the component ratios, therefore both the emulsion phase inversion and the phase inversion temperature were considered to fully explain their formation. Accordingly, it is hypothesized that the formation of these normal multiple emulsions is not a result of a temporary incompatibility (at the inversion point) during simple emulsion preparation, as previously reported. Rather, these normal W/O/Wm emulsions are a result of the simultaneous occurrence of catastrophic and transitional phase inversion processes. The formation of the primary emulsions (W/O) is in accordance with the Ostwald theory ,and the formation of the multiple emulsions (W/O/Wm) is in agreement with the Bancroft theory. (Link)
PREDICTING HYDROPHILIC DRUG ENCAPSULATION INSIDE UNILAMELLAR LIPOSOMES
A mathematical model has been developed to predict the encapsulation efficiency of hydrophilic drugs in unilamellar liposomes, and will be useful in formulation development to rapidly achieve optimized formulations. This model can also be used to compare drug encapsulation efficiencies of liposomes prepared via different methods, and will assist in the development of suitable process analytical technologies to achieve real-time monitoring and control of drug encapsulation during liposome manufacturing for hydrophilic molecules. Liposome particle size as well as size distribution, lipid concentration, lipid molecular surface area, and bilayer thickness were used in constructing the model. Most notably, a Log-Normal probability function was utilized to account for sample particle size distribution. This is important to avoid significant estimation error. The model-generated predictions were validated using experimental results as well as literature data, and excellent correlations were obtained in both cases. A Langmuir balance study provided insight regarding the effect of media on the liposome drug encapsulation process. The results revealed an inverse correlation between media ionic strength and lipid average molecular area, which helps to explain the phenomenon of inverse correlation between media ionic strength and drug encapsulation efficiency. Finally, a web application has been written to facilitate use of the model allowing calculations to be easily performed. This model will be useful in formulation development to rapidly achieve optimized formulation. (Link)