Joseph D. Gresser

Bioresorbable bone graft substitutes of different osteoconductivities: a histologic evaluation of osteointegration of poly(propylene glycol-co-fumaric acid)-based cement implants in rats.

Bioresorbable bone graft substitutes may significantly reduce the disadvantages associated with autografts, allografts and other synthetic materials currently used in bone graft procedures. We investigated the biocompatibility and osteointegration of a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene-glycol-co-fumaric acid), or simply poly(propylene fumarate), PPF, which is crosslinked in the presence of soluble and insoluble calcium filler salts. Four sets of animals each having three groups of 8 were evaluated by grouting bone graft substitutes of varying compositions into 3-mm holes that were made into the anteromedial tibial metaphysis of rats. Four different formulations varying as to the type of soluble salt filler employed were used: set 1--calcium acetate, set 2--calcium gluconate, set 3--calcium propionate, and set 4--control with hydroxapatite, HA, only. Animals of each of the three sets were sacrificed in groups of 8 at postoperative week 1, 3, and 7. Histologic analysis revealed that in vivo biocompatibility and osteointegration of bone graft substitutes was optimal when calcium acetate was employed as a soluble salt filler. Other formulations demonstrated implant surface erosion and disintegration which was ultimately accompanied by an inflammatory response. This study suggested that PPF-based bone graft substitutes can be designed to provide an osteoconductive pathway by which bone will grow in faster because of its capacity to develop controlled porosities in vivo. Immediate applicability of this bone graft substitute, the porosity of which can be tailored for the reconstruction of defects of varying size and quality of the recipient bed, is to defects caused by surgical debridement of infections, previous surgery, tumor removal, trauma, implant revisions and joint fusion. Clinical implications of the relation between developing porosity, resulting osteoconduction, and bone repair in vivo are discussed.

Effect of polymer foam morphology and density on kinetics of in vitro controlled release of isoniazid from compressed foam matrices.

The purpose of this study was to compare the effect of polymer foam morphology and density prior to compaction on the kinetics of isoniazid (INH) release from the final high-density extruded matrices. The feasibility of preparing low density foams of several biopolymers, including poly(L-lactide) (PLLA), poly(glycolide) (PGA), poly(DL-lactide-co-glycolide) (PLGA), poly(gamma-benzyl-L-glutamate) (PBLG), and poly(propylene fumarate) (PPF), via a lyophilization technique was investigated. Low-density foams of PLGA, PBLG, and a mixture of PLGA and PPF were successfully fabricated by lyophilization of the frozen polymer solutions either in glacial acetic acid or in benzene. The morphology of these foams depends on the polymer as well as the solvent used in the fabrication process. Thus, PLGA produces a capillary structure when lyophilized from benzene solution and a leaflet structure from glacial acetic acid, but PBLG yields a leaflet structure from benzene. Matrices were prepared by impregnating these foams with aqueous solutions of INH, removing the water by a second lyophilization, and then compressing the low-density INH containing foams by compaction and high-pressure extrusion. The resulting nonporous matrices had densities of approximately 1.30 g/cm3. In vitro kinetics were in accord with the Roseman-Higuchi diffusion model and demonstrate that release rates depend on the initial foam density, while foam structure has little influence on the release kinetics.

Controlled release of biologically active agents for purposes of agricultural crop management

Abstract Increasing attention is being directed to reducing the amount of pesticides, herbicides, and other biologically active agents used in modern agricultural crop management. One method for reducing the amount of such agents, while still maintaining effectiveness, is to encapsulate or otherwise incorporate the active agent into some from of plastic. Such ‘filled’ plastics, usually prepared by certain techniques for standard broadcast methods used in agriculture, may be sprayed, dusted, or spread as needed. By being incorporated into the plastic, the active agent diffuses slowly, but continuously, from the plastic matrix. It has been found in numerous instances that this use of controlled release delivery systems results in using less amount of the active agent. Further, with increasing attention being directed toward biologicals, rather than organic chemicals, for use in crop management, this incorporation of the biologicals into plastic serves the role of protection of the biological, as well as providing for slow release. One method of preparing an encapsulated polymeric controlled release system will be reviewed in depth, as well as field results.

Expression of liver-specific functions by rat hepatocytes seeded in treated poly(lactic-co-glycolic) acid biodegradable foams.

Techniques of liver replacement would benefit patients awaiting donor livers and may be a substitute for transplantation in patients whose livers can regenerate. Poly(lactic-co-glycolic acid) (PLGA) copolymers are biodegradable and have been shown to be useful as scaffolds for seeding and culturing various types of cells. In this study, foam disks were prepared from PLGA (lactic-to-glycolic mole ratio of 85:15) by lyophilization of benzene (5% w/v) solutions. These disks were then used as scaffolds for rat hepatocyte culture. Foams were coated with either a type I collagen gel (0.1% w/v), coated with gelatin (5% w/v), or treated with oxygen plasma (25 W, 90 s) to modify their surface chemistry and wettability. The disks were then seeded with rat hepatocytes (10(6)/mL) and cultured for a period of 2 weeks. All surface treatments resulted in increased hydrophilicity, the greatest being obtained by collagen treatment (contact angle < 10 degrees ), and a minimal decrease in void fraction (5%). DNA content after a 2-week culture period increased proportionally with the wettability of the treated foam surface. Urea synthesis in untreated foams averaged 15.3 +/- 2.3 microg/h/microg DNA, which was significantly higher than that for controls, whereas gelatin and collagen treated foams exhibited urea synthetic rates below the control levels at all times. The DNA content decreased significantly by about 50% between days 1 and 12. PLGA foams, treated and untreated, represent a promising scaffold for scaling up hepatocyte cultures.

Osteoconductivity of an injectable and bioresorbable poly(propylene glycol-co-fumaric acid) bone cement.

We have investigated an injectable form of a resorbable bone cement based on in situ crosslinking of the unsaturated polyester, poly(propylene glycol-co-fumaric acid) (PPF). This material, filled with calcium gluconate/hydroxyapatite (CG/HA), cures to a hard cement degradable by hydrolysis. The purpose of this study was to evaluate the osteoconductive properties of this injectable cement. The cement was used as an adjunct to fixation with an intramedullary rod in the rat femoral osteotomy model. Ingrowth of new bone into the cement was examined in vivo. Negative and positive controls with rigid and loose internal fixation were included for comparison. Animals were evaluated histologically and histomorphometrically at 4 weeks postoperatively. Results of this study showed osteoblastic activity and new bone formation at the interface between the femoral bone and the cement in the experimental group. However, there was little bone remodeling at the endosteal surface in positive and negative controls. Histologic evaluation of the cement revealed the formation of cavitations, which likely resulted from leaching of the highly soluble calcium gluconate portion of the filler from the cement. These cavitations were sites of ingrowth of vascular and bony tissues. Intimate contact between the bone cement and the endosteal surface of the cortex was found. Quantitative histomorphometric analysis corroborated these observations. Findings of this study demonstrated the osteoconductivity of this type of injectable PPF-based bone cement.

Developing porosity of poly(propylene glycol-co-fumaric acid) bone graft substitutes and the effect on osteointegration: A preliminary histology study in rats

Abstract -Bioresorbable bone graft substitutes could eliminate disadvantages associated with the use of autografts, allografts and other synthetic materials. We investigated a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene fumarate) which is crosslinked in the presence of soluble and insoluble calcium filler salts. This compact bone graft substitute material develops porosity in vivo by leaching of the soluble filler salts. In attempt to develop materials whose in vivo porosity can be designed such that implant degradation would occur at a rate that remains supportive of the overall structural integrity of the repairing defect site, we studied the early tissue response upon implantation in a bony defect. Three grout formulations of varying solubilities using slightly soluble hydroxyapatite (HA) and soluble calcium acetate (CA) were evaluated in 3 mm holes made in the anteromedial tibial metaphysis of 200 g Sprague Dawley rats (n = 16 per formulation for a total of 48 animals). Grout formulations cured in situ. Animals from each formulation were sacrificed in groups of 8 at 4 days and 3 weeks postoperatively. Histologic analysis of the healing process revealed improved in vivo osteointegration of bone graft substitutes when a higher loading of calcium acetate was employed. All formulations maintained implant integrity and did not provoke sustained inflammatory responses. This study suggested that the presence of a soluble salt permits in vivo development of porosity of a poly(propylene fumarate) based bone graft substitute material.

Design of capsules that burst at predetermined times by dialysis

Abstract This paper presents a mathematical model of a capsule or coating that ruptures after a predetermined time on exposure to water. A biodegradable capsule of this sort could be used to deliver pulsed doses of drugs (e.g. hormones) to patients, to release insecticides a certain time after a rainfall, etc. The model of a spherical capsule suggests that the time it takes to burst can be controlled successfully by the initial radius and wall thickness, provided a design criterion is met. If N 2 = P 0 r 0 /2 Ml 0 ( P 0 is the osmotic pressure of the contents, r 0 is the initial radius, M is the modulus of elasticity, and l 0 is the initial thickness of the capsule wall) is larger than two times N 1 = Y/M ( Y is the yield stress), the capsules bursting time will be approximately a linear function of the initial radius and a quadratic function of wall thickness. If N 2 N 1 , the bursting time is either radically sensitive to initial radius and wall thickness, or else the capsule does not burst.

Sustained release of a dual antimalarial system

The sustained release of a dual drug system in a biodegradable carrier was evaluated in rhesus monkeys and mice. The two drugs were sulphadiazine (WR‐7557) (3H‐labelled) and 2,4‐diamino‐6‐(2‐naphthylsulphonyl)quinazoIine (WR‐158122) (14C‐Iabelled). The carrier was a polymer of L‐lactide (90 parts by weight) and glycolide (10 parts), with a molecular weight of 46 000. The dual system injected was a blend of two preparations, each 50% (w/w) of the appropriate drug in the polymer, in a weight ratio of ten parts of the sulphadiazine system to one part of the WR‐158122 system. This blend, as cryogenically ground particles, was injected intramuscularly into three monkeys; a fourth monkey received an equivalent dose of a 10:1 mixture of the pure drugs. Excretion of radioactivity in urine and faeces was measured over 13 weeks. Similar studies were done with mice. In monkeys, 14C‐labelled material derived from WR‐158122 was excreted at a nearly uniform rate; expressed as WR‐158122, the rate of excretion from the pure drug mix was approximately 50 μg day−1 from week three to week 13, while the drug/polymer matrix released the drug at approximately 26 μg day−1 during this period. Recovery of tritiated materials derived from sulphadiazine was approximately 82% of the injected dose in the first three weeks and only a slight difference in release characteristics of the pure drug mix and the mixed drug/polymer matrices was observed. Results of studies with both mice and monkeys were generally consistent.

Larger animal testing of an injectable sustained release fertility control system.

Abstract Results are reported of the testing in four female baboons of an injectable fertility control system. This system, an injectable powder of 90–180μ particle size, contained 20% by weight of carbon-14 labeled norethisterone (NET) in a biodegradable tritium labeled polymer matrix. The polymer was synthesized from 90 parts by weight L-lactide and 10 parts glycolide to a molecular weight of 200,000. Results of the baboon tests indicate a uniform release with a pronounced but not dramatic early “burst” and an approximate system lifetime of two months. Concomitant release of polymer and drug was observed as measured by tritium and carbon-14 in the excreta. These data, the first for this injectable formulation in a large animal, were correlated with earlier results from smaller animals. Release rates were found to be strongly dependent on the quantity of drug injected. Based on this correlation it may now be projected that an injection of 150 mg NET will provide an average release of approximately 800 μg NET/day for 90 days.

Pulsed FSH release from an implantable capsule system

Abstract An implantable capsule capable of bursting at a predetermined time is proposed for pulsed delivery of hormones. This delivery system consists of a core of an effervescent agent enclosed in a hollow biodegradable poly(lactic-co-glycolic) acid (PLGA) polymer cylinder, closed at one end, and capped at the other end with a PLGA membrane. On immersion, entry of water through the membrane results in sufficient pressure increase (due to gas generation) to rupture (‘burst’) the membrane. The system, designed to deliver FSH, for specific application in cattle reproduction management, was tested in vitro and in vivo in rabbits. Burst times from 2 h to 168 h have been obtained in vitro compared with peak FSH levels observed from 6 to 64 h in vivo. Among the parameters investigated (membrane thickness, capsule size, and core composition), the core ingredient composition was the most influential factor in determining the burst time.