A model-dependent approach to correlate accelerated with real-time release from biodegradable microspheres

The purpose of this study was to determine the feasibility of applying accelerated in vitro release testing to correlate or predict long-term in vitro release of leuprolide poly(lactideco-glycolide) microspheres. Peptide release was studied using a dialysis technique at 37°C and at elevated temperatures (50°C–60°C) in 0.1 M phosphate buffered saline (PBS) pH 7.4 and 0.1 M acetate buffer pH 4.0. The data were analyzed using a modification, of the Weibull equation. Peptide release was temperature dependent and complete within 30 days at 37°C and 3 to 5 days at the elevated temperatures. In vitro release profiles at the elevated temperatures correlated well with release at 37°C. The shapes of the release profiles at all temperatures were similar. Using the modified Weibull equation, an increase in temperature was characterized by an increase in the model parameter, α, a scaling factor for the apparent rate constant. Complete release at 37°C was shortened from ∼30 days to 5 days at 50°C, 3.5 days at 55°C, 2.25 days at 60°C in PBS pH 7.4, and 3 days at 50°C in acetate buffer pH 4.0. Values for the model parameter β indicated that the shape of the release profiles at 55°C in PBS pH 7.4 (2.740) and 50°C in 0.1 M acetate buffer pH 4.0 (2.711) were similar to that at 37°C (2.577). The E_a for hydration and erosion were determined to be 42.3 and 19.4 kcal/mol, respectively. Polymer degradation was also temperature dependent and had an E_a of 31.6 kcal/mol. Short-term in vitro release studies offer the possibility of correlation with long-term release, thereby reducing the time and expense associated with longterm studies. Accelerated release methodology could be useful in the prediction of long-term release from extended release microsphere dosage forms and may serve as a quality control tool for the release of clinical or commercial batches. Selected for the 2005 AAPS Outstanding Graduate Student Research Award in Pharmaceutical Technologies Sponsored by Solvay Pharmaceuticals.     

Preparation of biodegradable microspheres and matrix devices containing naltrexone

In this study, the use of biodegradable polymers for microencapsulation of naltrexone using solvent evaporation technique is investigated. The use of naltrexone microspheres for the preparation of matrix devices is also studied. For this purpose, poly(L-lactide) (PLA) microspheres containing naltrexone prepared by solvent evaporation technique were compressed at temperatures above the Tg of the polymer. The effect of different process parameters, such as drug/polymer ratio and stirring rate during preparation of microspheres, on the morphology, size distribution, and in vitro drug release of microspheres was studied. As expected, stirring rate influenced particle size distribution of microspheres and hence drug release profiles. By increasing the stirring speed from 400 to 1200 rpm, the mean diameter of microspheres decreased from 251 μm to 104 μm. The drug release rate from smaller microspheres was faster than from larger microspheres. However, drug release from microspheres with low drug content (20% wt/wt) was not affected by the particle size of microspheres. Increasing the drug content of microspheres from 20% to 50% wt/wt led to significantly faster drug release from microspheres. It was also shown that drug release from matrix devices prepared by compression of naltrexone microspheres is much slower than that of microspheres. No burst release was observed with matrix devices. Applying higher compression force, when compressing microspheres to produce tablets, resulted in lower drug release from matrix devices. The results suggest that by regulating different variables, desired release profiles of naltrexone can be achieved using a PLA microparticulate system or matrix devices.     

Development of a dialysis in vitro release method for biodegradable microspheres

The purpose of this research was to develop a simple and convenient in vitro release method for biodegradable microspheres using a commercially available dialyzer. A 25 KD MWCO Float-a-Lyzer was used to evaluate peptide diffusion at 37°C and 55°C in different buffers and assess the effect of peptide concentration. In vitro release of Leuprolide from PLGA microspheres, having a 1-month duration of action, was assessed using the dialyzer and compared with the commonly used “sample and separate” method with and without agitation. Peptide diffusion through the dialysis membrane was rapid at 37°C and 55°C in all buffers and was independent of peptide concentration. There was no detectable binding to the membrane under the conditions of the study. In vitro release of Leuprolide from PLGA microspheres was tri-phasic and was complete in 28 days with the dialysis technique. With the sample and separate technique, linear release profiles were obtained with complete release occurring under conditions of agitation. Diffusion through the dialysis membrane was sufficiently rapid to qualify the Float-a-Lyzer for an in vitro release system for microparticulate dosage forms. Membrane characteristics render it useful to study drug release under real-time and accelerated conditions. Published: October 6, 2005     

A novelin vitro release technique for peptide-containing biodegradable microspheres

The purpose of this study was to develop and evaluate a dialysisin vitro release technique for peptide-containing poly(d, 1-lactide-coglycolide) (PLGA) microspheres (ms) that would correlate within vitro data. Using a luteinizing hormone- releasing hormone analogue (LHRH), Orntide acetate, solubility and stability were determined in 0.1 M phosphate buffer (PB), pH 7.4, and in 0.1 M acetate buffer (AB), pH 4.0, with highperformance liquid chromotography (HPLC), and peptide permeability through a dialysis membrane (molecular weight cut-off 300,000) was determined. Orntide ms were prepared by a dispersion/solvent extraction/evaporation method and characterized for drug content (HPLC), particle size distribution (laser diffraction method), and surface morphology (scanning electron microscopy).In vitro release was studied in PB using a conventional extraction method and with a new dialysis method in AB. Gravimetric analyses of polymer mass loss and matrix hydration, and peptide adsorption to blank PLGA ms (50∶50, M_w 28 022) were carried out in PB and AB upon incubation at 37°C. Serum Orntide and testosterone levels in rats after administration of Orntide ms were determined by radioimmunoassay. Orntide acetate solubility was influenced by pH; approximately 2.3 mg/mL dissolved in PB and >18 mg/mL in AB. Stability was pH- and temperature-dependent. The peptide was very stable at pH 4.0, 4°C, but degraded rapidly at pH 7.4,37°C. Peptide permeability through the dialysis membrane was accelerated by agitation and>95% equilibrium was reached within 48 hours. The overall release rate was higher with the dialysis method. Mass loss of the Orntide ms was faster in AB (50% loss in 3 weeks: 95% in 35 days) than in PB (65% in 35 days). In contrast, hydration after 35 days was 4-fold higher in PB. The nonspecific adsorption to blank ms was greater in PB (128 μ g Orntide/10 mg PLGA) compared with AB (<5 μ g Orntide/10 mg PLGA). Administration of 30-day Orntide PLGA ms to rats resulted in an initial serum Orntide level of 21 ng/mL after 6 hours and a C_max of 87 ng/mL after 6 days. Testesterone levels were suppressed immediately after ms administration (3 mg Orntide/Kg) from 5.2 ng/mL to 0.3 ng/mL (after 24 hours) and remained suppressed for 38 days. Orntide acetate solubility and degradation kinetics were markedly influenced by pH of the buffer systems and mass loss; matrix hydration, as well as the nonspecific adsorption to blank ms, was pH-dependent. Thein vitro release profile obtained with the dialysis method in AB correlated well with thein vivo data, therepy providing a more reliable prediction ofin vivo performance.     

Effect of isopropyl myristic acid ester on the physical characteristics and in vitro release of etoposide from PLGA microspheres

The purpose of this paper was to study the effect of the isopropyl myristic acid ester (IPM) on the physicochemical characteristics of etoposide-loaded poly(lactic-co-glycolic acid) (PLGA) microspheres-specifically, the effects on the size and drug loading of the microspheres, the polymer matrix and surface morphology, and the release of etoposide from the microspheres. The experiment was structured to examine 2 IPM concentrations (25% and 50%) and 1 control (no IPM) at 2 different etoposide-loading percentages (10% and 5%). The microspheres were prepared using a single-emulsion solvent-extraction procedure. Samples from each batch of microspheres were then analyzed for size distribution. drug-loading efficiency, surface characteristics, in vitro release, and in vitro microsphere degradation. The incorporation of 50% IPM significantly increased (P<05) the size of the microspheres when compared with the control and 25% IPM microspheres. However, incorporation of 25% or 50% IPM did not change (P>.05) the drug-loading efficiency in comparison with the microspheres prepared without IPM. The microspheres containing 50% IPM were shown to significantly increase (P<.05) the release of etoposide from the microspheres at both etoposide concentrations. The microspheres prepared incorporating 25% IPM and 5% etoposide increased the in vitro release (P<.05) in comparison with the microspheres prepared without IPM. The 5% etoposide-PLGA microspheres showed a smooth, nonporous surface that changed to a dimpled. nonporous surface after addition of 25% IPM. During the in vitro degradation study, the IPM-containing microspheres slowly became porous but retained their structural integrity throughout the experiment.     

磁性顺铂微球的药物控释研究

目的 :改善磁性顺铂微球的药突释和滞释 ,实现控释。方法 :用不同的工艺制备磁性顺铂微球并进行药物释放的体外、体内测定。结果 :当高分子基质材料中疏水性骨架材料 ,含有水解键的交联偶合材料 =7:3、搅拌速度 1 5 0 0r.min- 1 ,成型温度 2 0℃时 ,制备的磁性顺铂微球具有较好的控释特性。结论 :对开发磁性微球和导向治疗恶性肿瘤有一定意义。     

Synthesis and characterization of cross-linked malonylchitosan microspheres for controlled release of acyclovir

The purpose of the present study was to obtain a polymeric system for delayed release of the drug acyclovir (ACV), which can be used for treatment of Herpes simplex and Varicella Zoster. The gelled chitosan (GCT) microspheres were obtained by coacervation-phase separation. They were treated with malonic acid to obtain malonylchitosan (MLCT) microspheres, which were characterized by, Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (¹³C NMR), elemental analysis (CHN), thermogravimetric analysis (TG/DTG) and scanning electron microscopy (SEM). The drug was encapsulated in MLCT microspheres by a contact adsorption technique, and the final formulation (MLCT-ACV), was analyzed for loading efficiency, degree of swelling and in vitro release profiles. The results obtained support the N-substitution of malonyl groups in the MLCT microspheres. The loading efficiency increased with impregnation time and a major amount of drug was encapsulated after 24 h. The swelling rate was higher in acid pH. The median release time was 5.5 h in pH 1.2 and 6.8. The mechanism involved in release was non-Fickian (0.43 < n < 0.85, n = 0.8474) and Super Case II kinetics (n > 1, n = 1.0491) at pH 1.2 and 6.8, respectively.     

Preparation,in vitro release,in vivo absorption and biocompatibility studies of insulin-loaded microspheres in rabbits

The purpose of this study was to develop a single-dose insulin delivery system based on poly (lactide-co-glycolide) (PLGA) microspheres to provide basal insulin level for a prolonged period. Insulin-loaded PLGA microspheres were prepared by water-in-oil-in-water double emulsion (batch A) and solid-in-oil-in-water emulsion (batch B) methods. Microspheres were characterized for physical characteristics and in vitro release. In vivo absorption of insulin and biocompatibility of insulin-loaded PLGA microspheres were performed in diabetic New Zealand white rabbits. Light and transmission electron microscopy were performed on the skin tissues excised from microspheres injected sites in order to study the biocompatibility. The burst release of insulin was high (47%) from batch B and low (5%) from batch A. Therefore, we mixed microspheres of batch A and B in ratio of 3:1 w/w, which produced desirable in vitro release profile. In vivo absorption study showed that insulin-loaded microspheres provided a serum insulin level of 20-40 microU/ml up to 40 days. Biocompatibility study provided evidence of normal inflammatory and foreign body reactions, which were characterized by the presence of macrophages, fibroblasts and foreign body giant cells. Neither necrosis nor tissue damage was identified. At the end of 12 weeks, no distinct histological differences were observed in comparison to the control tissue samples. In conclusion, insulin-loaded PLGA microspheres controlled the in vivo absorption of insulin to maintain the basal insulin level for longer period and the delivery system was biocompatible.     

Poly(D,L-Lactide-Co-Glycolide) microspheres containing 5-fluorouracil: Optimization of process parameters

The objective of this research was to optimize the processing parameters for poly(D,L-lactide-coglycolide) (PLGA) microspheres of 5-fluorouracil (5-FU) and to mathematically relate the process parameters and properties of microspheres. Microspheres were prepared by a water-in-oil-in-water emulsion solvent evaporation technique. A 3² factorial design was employed to study the effect of the volume of the internal phase of the primary emulsion and the volume of the external phase of the secondary emulsion on yield, particle size, and encapsulation efficiency of microspheres. An increase in the volume of the internal phase of the primary emulsion resulted in a decrease in yield and encapsulation efficiency and an increase in particle size of microspheres. When the volume of the external phase of the secondary emulsion was increased, a decrease in yield, particle size, and encapsulation efficiency was observed. Microspheres with good batch-to-batch reproducibility could be produced. Scanning electron microscopic study indicated that microspheres existed as aggregates.     

Evaluation of orntide microspheres in a rat animal model and correlation to in vitro release profiles

Orntide acetate, a novel luteinizing hormone-releasing hormone (LHRH) antagonist, was prepared and evaluated in vivo in 30-day and 120-day sustained delivery formulations using a rat animal model. Orntide poly(d,l- lactide-co-glycolide) (PLGA) and poly(d,l- lactide) (PLA) microspheres were prepared by a dispersion method and administered subcutaneously in a liquid vehicle to rats at 2.2 mg Orntide/kg of body weight (30-day forms) or 8.8 mg Orntide/kg (120-day forms). Serum levels of Orntide and testosterone were monitored by radioimmunoassays, and a dose-response study at 4 closes (3, 2.25, 1.5, and 1.75 mg Orntide/kg) was conducted to determine the effective dose of Orntide. Microspheres with diameters between 3.9 and 14 μ were prepared. The onset and duration of testosterone suppression varied for different microsphere formulations and were influenced both by polymer properties and by microsphere characteristics. Microspheres prepared with 50∶50 and 75∶25 copolymers effectively sustained peptide release for 14 to 28 days, whereas an 85∶15 copolymer and the PLA microspheres extended the pharmacological response for more than 120 days. Increase in drug load generally accelerated peptide release from the microspheres, resulting in higher initial serum levels of Orntide and shorter duration of the release: In general, apparent release was faster in vivo than under in vitro conditions. Orntide microspheres effectively suppressed testosterone in rats, providing rapid onset of release and extended periods of chemical castration. Testosterone suppression occurred immediately after microsphere administration without the initial elevation seen with LHRH superagonists.     

Long-term release and improved intracellular penetration of oligonucleotide-polyethylenimine complexes entrapped in biodegradable microspheres

The aim of this work was to design a biodegradable delivery system for oligonucleotides providing both a sustained release and an improved intracellular penetration. To this purpose oligonucleotide/polyethylenimine (ON/PEI) complexes at nitrogen to phosphate (N/P) molar ratios of about 15 or 40 were encapsulated into poly(lactide-co-glycolide) microspheres by the multiple emulsion-solvent evaporation technique. ON/PEI complexes were efficiently entrapped inside microspheres. The introduction of salts within the external aqueous phase allowed an improvement of microsphere characteristics. In particular, the use of sodium chloride led to a reduced microsphere porosity and a more homogeneous ON distribution inside the polymeric matrix. These effects were attributed to the reduced flux of water from the external aqueous phase toward the internal aqueous droplets, due to the osmotic effect of sodium chloride. Both, the reduced porosity and the improved ON distribution inside the matrix, were considered responsible for the lower burst effect and the slower ON release rate from microsphere prepared with sodium chloride. ON/PEI complexes encapsulated inside microspheres were also protected toward enzymatic degradation in fetal calf serum. Interestingly, ON/PEI complexes slowly released from microspheres efficiently penetrated inside HeLa cells and oligonucleotides were preferentially located in the nucleus.     

微球包裹pcDNA3-GRF（1-32）在小鼠肌肉组织的表达及对生长的影响

以乳酸-乙醇酸共聚物为载体,采用复乳液中蒸发法制备载生长激素释放因子（Growth hormone releasing factor,GRF）真核表达质粒pcDNA3-GRF（1-32）微球,其中包封率达69%,平均粒径为2.20μm,载药量80μg/mg,收率70%;体内转染小鼠肌肉组织, 提取注射部位肌肉组织DNA 和总RNA,经PCR和RT-PCR,发现注射pcDNA3-GRF（1-32）微球组的GRF表达水平最高（UVIBAND Version 99分析）,释放的GRF表达质粒在小鼠肌肉内存在并表达的时间至少30d。体重统计结果表明,30d后微球包裹质粒组累积增重与其它组相比差异极显著（P<0.01）,比裸质粒组、质粒和空白微球混合物组、生理盐水组分别高12.87%,19.72%,58.58%;结论认为,载pcDNA3-GRF（1-32）微球具有缓释作用,并可实现GRF基因体内局部基因转染、表达,发挥其相应的生物学效应,有希望成为提高质粒在动物肌肉组织表达效率的新方法。     

Identification of chemically modified peptide from poly(D,L-lactide-co-glycolide) microspheres under in vitro release conditions

The purpose of this research was to study the chemical reactivity of a somatostatin analogue octreotide acetate, formulated in microspheres with polymers of varying molecular weight and co-monomer ratio under in vitro testing conditions. Poly(D,L-lactide-co-glycolide) (PLGA) and poly(D,L-lactide) (PLA) microspheres were prepared by a solvent extraction/evaporation method. The microspheres were characterized for drug load, impurity content, and particle size. Further, the microspheres were subjected to in vitro release testing in acetate buffer (pH 4.0) and phosphate buffered saline (PBS) (pH 7.2). In acetate buffer, 3 microsphere batches composed of low molecular weight PLGA 50∶50, PLGA 85∶15, and PLA polymers (≤10 kDa) showed 100% release with minimal impurity formation (<10%). The high molecular weight PLGA 50∶50 microspheres (28 kDa) displayed only 70% cumulative release in acetate buffer with significant impurity formation (∼24%). In PBS (pH 7.4), on the other hand, only 50% release was observed with the same low molecular weight batches (PLGA 50∶50, PLGA 85∶15, and PLA) with higher percentages of hydrophobic impurity formation (ie, 40%, 26%, and 10%, respectively). In addition, in PBS, the high molecular weight PLGA 50∶50 microspheres showed only 20% drug release with ∼60% mean impurity content. The chemically modified peptide impurities inside microspheres were structurally confirmed through Fourier transform-mass spectrometry (FT-MS) and liquid chromatography/mass spectrometry (LC-MS/MS) analyses after extraction procedures. The adduct compounds were identified as covalently modified conjugates of octreotide with lactic and glycolic acid monomers within polymeric microspheres. The data suggest that due to steric hindrance factors, polymers with greater lactide content were less amenable to the formation of adduct impurities compared with PLGA 50∶50 copolymers.     

Influence of aluminum tristearate and sucrose stearate as the dispersing agents on physical properties and release characteristics of Eudragit RS microspheres

The purpose of this research was to investigate the effects of different concentrations of polymer and sucrose stearate, aluminum tristearate as dispersing agents on microsphere properties and performance. The yield values of microspheres were over the 78%, and the encapsulation efficiencies were found to be ∼735. Particle sizes of microspheres prepared with aluminum tristearate were between 76 and 448 μm, while that of the microspheres containing sucrose stearate were between 521 and 2000 μm. Morphological and physicochemical properties of microspheres were investigated by scanning electron micrography and differential scanning calorimetry (DSC). DSC analysis indicated that verapamil hydrochloride formed a solid solution with acrylic polymers. In vitro release studies were performed using the flow-through cell method. While ∼80% of drug was released from the microspheres containing aluminum tristearate in 480 minutes, the same amount of drug was released from microspheres containing sucrose stearate in only 60 minutes. Chemical structures and concentrations of the dispersing agents were clearly effective on the physical properties of microspheres and their drug-release characteristics. Published: February 24, 2006     

Effect of additives on the release of a model protein from PLGA microspheres

The purpose of this study was to investigate the effect of 2 additives, poly(ethylene glycol (PEG) 1000 and 1,2,3-tridecanoyl glycerol (tricaprin), on the physico-chemical characteristics and in vitro release of a model protein, bovine serum albumin (BSA), form poly(D,L-lactic-co-glycolic acid) (PLGA) microspheres. BSA-loaded microspheres were prepared by the double emulsion solvent evaporation method. Additives were incorporated into microspheres to modify the release of protein. The addition of PEG 1000 and tricaprin changed the surface characteristics of microspheres from smooth and nonporous to porous and dimpled, respectively. The in vitro release profiles showed that the additives significantly (P<0.05) increased the early-stage release of BSA from microspheres.     

In vivo release kinetics of octreotide acetate from experimental polymeric microsphere formulations using oil/water and oil/oil processes

The purpose of the present study was to characterize the in vivo release kinetics of octreotide acetate from microsphere formulations designed to minimize peptide acylation and improve drug stability. Microspheres were prepared by a conventional oil/wate (o/w) method or an experimental oil/oil (o/o) dispersion technique. The dosage forms were administered subcutaneously to a rat animal model, and serum samples were analyzed by radioimmunoassay over a 2-month period. An averaged kinetic profile from each treatment group, as a result, was treated with fractional differential equations. The results indicated that poly(l-lactide) microspheres prepared by the o/o dispersion technique provided lower area under the curve (AUC) values during the initial diffusion-controlled release phase, 7.79 ng×d/mL, versus 75.8 ng/sxd/mL for the o/w batch. During the subsequent erosion-controlled release phase, on the other hand, the o/o technique yielded higher AUC values, 123 ng×d/mL, versus 42.2 ng×d/mL for the o/w batch. The differences observed between the 2 techniques were attributed to the site of drug incorporation during the manufacturing process, given that microspheres contain both porous hydrophilic channels and dense hydrophobic matrix regions. An o/o dispersion technique was therefore expected to produce microspheres with lower incorporation in the aqueous channels, which are responsible for diffusion-mediated drug release.     

Preparation, characterization, and in vivo evaluation of salmon calcitonin microspheres

Purpose. This study was done to prepare, characterize, and evaluate salmon calcitonin (sCT) microspheres (ms) in vivo using a low molecular weight, hydrophilic 50∶50 poly (D,L-lactide-co-glycolide) polymer (PLGA).Methods. sCT ms were prepared by a dispersion/solvent extraction/evaporation process and characterized for drug content, particle size, surface morphology, and structural integrity of encapsulated peptide. Peptide stability and binding to the polymer was studied in 0.1 M phosphate buffer (PB), pH 7.4, and 0.1 M acetate buffer (AB), pH 4.0. Serum sCT levels were monitored for 2 weeks after subcutaneous injection of sCT ms to rats.Results. sCT ms were essentially free of discernible surface pores with a particle size distribution in the range of 16 to 89 mm and mean particle size of 51 and 53 mm for 2 batches. Fourier Transform Matrix-assisted Laser Desorption mass spectrometry of the extracted peptide showed that the encapsulation process did not alter its chemical structure. The peptide was substantially more stable in AB than in PB. Peptide binding to the polymer was dependent on pH and was markedly higher in PB than in AB. In vivo study proved that elevated serum sCT levels could be sustained for at least 10 days after administration of sCT ms to rats at a dose of 1.0 mg/kg.Conclusions. It was demonstrated that sCT could be incorporated into polymeric ms prepared from a low molecular weight, hydrophilic PLGA using a dispersion technique without altering molecular structure. A 2-week formulation was prepared at a dose of 1.0 mg/kg.     

Solvent treatment for β-d-galactosidase release from yeast cells

A method is presented for the release of β-d-galactosidase (β-d-galactoside galactohydrolase, EC 3.2.1.23) from yeast cells. Enzyme release is attained by mixing yeast cells with concentrated solvents (20 to 95%) and subsequently suspending and agitating the cells in buffer. Many solvents, including isopropanol, ethanol and methanol, were found to be effective. Enzyme release into buffer was relatively slow: 10–20 h was required for maximum yields. The release of protease and β-d-galactosidase was monitored. β-d-Galactosidase solubilization was achieved in high yield: 90% of the intracellular enzyme was released into the buffer. Because this method exhibits resistance to yield loss due to microbial degradation and is not sensitive to small changes in solvent in buffer concentration or treatment time, it is particularly suited to industrial-scale enzyme recoveries.     

Cytokine immunotherapy of cancer with controlled release biodegradable microspheres in a human tumor xenograft/SCID mouse model

 A novel biodegradable poly(lactic acid) microsphere formulation was evaluated for in vivo cytokine immunotherapy of cancer in a human tumor xenograft/severe combined immunodeficiency (SCID) mouse model. Co-injection of interleukin-2 (IL-2)-loaded microspheres with tumor cells into a subcutaneous site resulted in the complete suppression of tumor engraftment in 80% of animals. In contrast, bovine-serum-albumin(BSA)-loaded particles or bolus injections of poly(ethylene glycol)/IL-2 were ineffective in preventing tumor growth. The antitumor effect of IL-2 released by the microspheres was shown to be mediated by the mouse natural killer cells. This is the first evidence that the rejection of human tumor xenografts can be provoked by the sustained in vivo delivery of IL-2 from biodegradable microspheres. The use of poly(lactic acid) microspheres to deliver cytokines to the tumor environment could provide a safer and simpler alternative to gene therapy protocols in the treatment of cancer. Received: 9 September 1997 / Accepted: 30 October 1997