Upkar Bhardwaj

A Review of the Biocompatibility of Implantable Devices: Current Challenges to Overcome Foreign Body Response

In recent years, a variety of devices (drug-eluting stents, artificial organs, biosensors, catheters, scaffolds for tissue engineering, heart valves, etc.) have been developed for implantation into patients. However, when such devices are implanted into the body, the body can react to these in a number of different ways. These reactions can result in an unexpected risk for patients. Therefore, it is important to assess and optimize the biocompatibility of implantable devices. To date, numerous strategies have been investigated to overcome body reactions induced by the implantation of devices. This review focuses on the foreign body response and the approaches that have been taken to overcome this. The biological response following device implantation and the methods for biocompatibility evaluation are summarized. Then the risks of implantable devices and the challenges to overcome these problems are introduced. Specifically, the challenges used to overcome the functional loss of glucose sensors, restenosis after stent implantation, and calcification induced by implantable devices are discussed.

Controlled release of dexamethasone from PLGA microspheres embedded within polyacid-containing PVA hydrogels

The development of zero-order release systems capable of delivering drug(s) over extended periods of time is deemed necessary for a variety of biomedical applications. We hereby describe a simple, yet versatile, delivery platform based on physically cross-linked poly(vinyl alcohol) (PVA) microgels (cross-linked via repetitive freeze/thaw cycling) containing entrapped dexamethasone-loaded poly(lacticco-glycolic acid) (PLGA) microspheres for controlled delivery over a 1-month period. The incorporation of polyacids, such as humic acids, Nafion, and poly(acrylic acid), was found to be crucial for attaining approximately zero-order release kinetics, releasing 60% to 75% of dexamethasone within 1 month. Microspheres alone entrapped in the PVA hydrogel resulted in negligible drug release during the 1-month period of investigation. On the basis of a comprehensive evaluation of the structure-property relationships of these hydrogel/microsphere composites, in conjunction with their in vitro release performance, it was concluded that these polyacids segregate on the PLGA microsphere surfaces and thereby result in localized acidity. These surface-associated polyacids appear to cause acid-assisted hydrolysis to occur from the surface inwards. Such systems show potential for a variety of localized controlled drug delivery applications such as coatings for implantable devices.

Controlling Acute Inflammation with Fast Releasing Dexamethasone-PLGA Microsphere/PVA Hydrogel Composites for Implantable Devices

Background: Continuous release of dexamethasone from PLGA microsphere/PVA hydrogel composites has been shown to suppress the inflammatory tissue reaction in response to subcutaneously implanted foreign material for a period of one month. The scope of the present work is to investigate whether suppressing the initial acute inflammatory phase with fast releasing dexamethasone-PLGA microsphere/PVA composites (that release the drug over a period of one week) would prevent the development of a foreign body reaction in response to implantation in the subcutaneous tissue using a rat model. Methods: Dexamethasone loaded PLGA microspheres were prepared using the solvent evaporation method. In vitro release from microspheres was analyzed using USP apparatus 4 in phosphate buffered saline (PBS) at 37°C. Composites were fabricated in 18G needles by freeze-thaw cycling the PVA/microsphere dispersion. The composites were implanted in the subcutaneous tissue of anesthetized rats. The pharmacodynamic effect was evaluated by histological examination of the tissue surrounding the composites at pre-determined time points. Results: In vitro release studies showed that most of the drug entrapped in the microspheres was released within one week. At days 3 and 8, these fast releasing dexamethasone containing composites suppressed the acute phase of inflammation but did not prevent the development of an inflammatory reaction after dexamethasone was completely released from the composites. By day 30, chronic inflammation and fibrosis were observed in the tissue surrounding the drug-containing composites. On days 3 and 8, the number of inflammatory cells in the vicinity of the dexamethasone containing composites was similar to that in normal tissue. However, the number of inflammatory cells was higher in drug-containing composites as compared to drug-free composites by day 30. This was due to the inflammation being in a more advanced stage in drug-free composites where a granulomatous reaction had already developed. Conclusion: Fast release of dexamethasone from PLGA/PVA composites did not provide long-term protection against the foreign body reaction in response to implantation. It would appear that a sustained delivery of anti-inflammatory agents such as dexamethasone is necessary to suppress inflammation throughout the implant life-time.

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.

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.

Comparison of in vitro–in vivo release of Risperdal® Consta® microspheres

The objective was to investigate the relationship between in vitro and in vivo release of commercial Risperdal(®) Consta(®) microspheres. A modified USP apparatus 4 method was used for accelerated and real-time in vitro release testing. The in vivo plasma profile (clinical data) reported for the product was deconvoluted for comparison with the in vitro release profiles. The in vivo profile differed from the real-time in vitro profile and was faster initially and then slower after approximately 30 days. This effect is considered to be due to differences in the in vivo conditions such as small interstitial volume, low pH and immune response. Accelerated in vitro release profiles obtained at temperatures (50°C and 54.5°C) above the microsphere glass transition temperature (Tg∼48°C) overlapped with the in vivo profile after time scaling. A linear in vitro-in vivo relationship was observed with correlation coefficients of 0.97 and 0.99 at 50°C and 54.5°C, respectively. The accelerated test performed below the Tg had a similar release profile to that of the real-time in vitro test. The accelerated tests performed above the Tg of the microspheres showed the potential to be used for in vivo performance prediction as well as for quality control purposes.

A review of the development of a vehicle for localized and controlled drug delivery for implantable biosensors.

A major obstacle to the development of implantable biosensors is the foreign body response (FBR) that results from tissue trauma during implantation and the continuous presence of the implant in the body. The in vivo stability and functionality of biosensors are compromised by damage to sensor components and decreased analyte transport to the sensor. This paper summarizes research undertaken by our group since 2001 to control the FBR toward implanted sensors. Localized and sustained delivery of the anti-inflammatory drug, dexamethasone, and the angiogenic growth factor, vascular endothelial growth factor (VEGF), was utilized to inhibit inflammation as well as fibrosis and provide a stable tissue-device interface without producing systemic adverse effects. The drug-loaded polylactic-co-glycolic acid (PLGA) microspheres were embedded in a polyvinyl alcohol (PVA) hydrogel composite to fabricate a drug-eluting, permeable external coating for implantable devices. The composites were fabricated using the freeze-thaw cycle method and had mechanical properties similar to soft body tissue. Dexamethasone-loaded microsphere/hydrogel composites were able to provide anti-inflammatory protection, preventing the FBR. Moreover, concurrent release of dexamethasone with VEGF induced neoangiogenesis in addition to providing anti-inflammatory protection. Sustained release of dexamethasone is required for the entire sensor lifetime, as a delayed inflammatory response developed after depletion of the drug from the composites. These studies have shown the potential of PLGA microsphere/PVA hydrogel-based composites as drug-eluting external coatings for implantable biosensors.