The purpose of this study was to examine the degradative effect of weakly basic nucleophilic drugs on a lactide-co-glycolide (PLGA) polymer in a microsphere formulation. Biodegradable PLGA microspheres of two second-generation atypical antipsychotics, Risperidone and Olanzapine, were manufactured using a solvent extraction/evaporation technique. The effect of drug content, buffer pH and temperature on polymer molecular weight and degradation, were examined via a series of experiments and compared against a control (Placebo PLGA microspheres). In comparison to Placebo microspheres, significant polymer molecular weight reduction was observed upon encapsulation of varying levels of either Risperidone or Olanzapine. There was excellent correlation between the extent of molecular weight reduction during manufacture and the amount of encapsulated drug in the microspheres. Subsequent studies on polymer degradation showed: the following (a) the Placebo and Olanzapine microspheres followed pseudo first order kinetics, (b) Risperidone microspheres exhibited biphasic degradation profiles, and (c) polymer degradation was dependent on temperature, not pH. The findings of these studies show that encapsulation of weakly basic nucleophile type drugs into PLGA can accelerate the biodegradation of the PLGA and have major implications on the design of polymeric microsphere drug delivery systems.
Considerable advances in the areas of research involving biodegradable polymers as implantable reservoirs for sustained-release drug delivery have been realized over the past few decades [
Generally, degradation of poly(esters) like PLA and PLGA occurs in four major stages: polymer hydration causing disruption of the primary and secondary structure due to hydrogen bonding and van der Waals forces, loss of mechanical strength caused by the rupture of covalent bonds of the polymer backbone to form oligomers with acidic end-groups, polymer mass loss due to diffusion of acidic oligomers resulting in accelerated water absorption, polymer dissolution and/or phagocytosis [
As would be expected, polymer hydration of PLA or PLGA causes ester hydrolysis that has been shown to proceed via pseudo first order kinetics and is described as follows:
In addition to their use in surgical sutures and prosthetic devices, biodegradable poly(ester) polymers are also used in commercially available injectable products, for example, Risperdal Consta (Risperidone in PLGA) and Lupron Depot (Leuprolide acetate in PLGA) [
While the chemical composition of PLGA and PLA is essential for biodegradability and biocompatibility, a notable effect on the drug release profile is detected especially if the therapeutic agent is a basic drug. Such types of drugs are expected to interact with the acidic polymer thereby accelerating polymer degradation and influencing drug release from the polymer matrix. For instance, researchers have reported that tertiary amine drugs or nucleophilic compounds like local anesthetics accelerate the hydrolytic degradation of poly(D,L-lactide) polymer [
Towards this goal, the research presented in this publication explores the degradation in microsphere formulations containing basic drugs. The drugs chosen for the study are two popular second-generation atypical antipsychotics, Risperidone and Olanzapine. Both molecules are commonly prescribed for the treatment of schizophrenia and other psychotic disorders [
Chemical structures of Risperidone and Olanzapine.
The objectives of this study include: identification of any possible effects of Risperidone and Olanzapine on the PLGA polymer, assessment of the influence of drug loading on polymer degradation, determination of the impact of buffer and temperature on the polymer degradation process.
The study also highlights the similarities and differences in behavior between Olanzapine and Risperidone and provides a framework for researchers investigating sustained dosage formulations that utilize biodegradable polymers.
Hydrochloride salts of Risperidone and Olanzapine were purchased from Cipla Ltd., Bombay, India, DL65 : 35 PLGA (
Placebo, Risperidone, and Olanzapine PLGA microspheres were prepared by dispersing a homogenous solution of polymer and drug into an aqueous solution containing 0.35% Polyvinyl alcohol followed by solvent extraction/evaporation for 2 hours [
A 10 mg amount of microspheres was dissolved in 8 mL Acetonitrile followed by the addition of 32 mL of 0.1 M acetate buffer, pH 4.0. The contents were diluted with 80 : 20 Acetate buffer : acetonitrile mixture and filtered using 0.45
Particles were sized by laser diffractometry using a Malvern 2600 laser sizer (Malvern 2600 particle sizer, Malvern, UK). The average particle size was expressed as the volume mean diameter (
50 mg of drug loaded microspheres or Placebo microspheres was transferred to a bottle containing 50 mL of 0.02 M phosphate buffer saline (PBS) pH 7.2 containing 0.05% Tween 80, at 37°C. At predetermined intervals, the remaining microspheres were filtered using 0.65
The molecular weight distribution of Placebo and drug loaded microspheres was determined by gel permeation chromatography (GPC). The GPC system consisted of two Ultrastyragel columns connected in series (7.8 × 300 mm each, one with 104 Å pores and one with 103 Å pores), a delivery device (Shimadzu LC-6A, Japan), UV detector set at MWinitial is the initial polymer molecular weight (75 kDa),
Drug content for all the batches of Risperidone and Olanzapine microspheres was assessed by HPLC and the results are described in Table
Mean particle size of Risperidone and Olanzapine microspheres.
Drug loading (%) | Risperidone microspheres ( |
Olanzapine microspheres ( |
---|---|---|
1 | 8.80 | 10.4 |
5 | 12.7 | 9.80 |
10 | 15.4 | 12.3 |
15 | 23.4 | 10.0 |
35 | 20.8 | — |
Results of particle size analysis for Risperidone and Olanzapine microspheres, as measured by laser diffractometry, are listed in Table
The effect of 1–35% Risperidone loading on degradation of the 65 : 35 PLGA polymer in 0.02 M phospahte buffer saline (PBS) pH 7.2 containing 0.05% Tween 80, at 37°C, is presented as a semilogarithmic plot in Figure
Effect of Risperidone loading on polymer degradation.
Results from this study indicate that Risperidone loading had a profound effect on polymer molecular weight reduction at two distinct stages: after manufacture of the microspheres ( within 2 days of incubation in PBS (
The impact of Olanzapine on degradation of the 65 : 35 PLGA polymer in 0.02 M phosphate buffer saline (PBS) pH 7.2 containing 0.05% Tween 80, at 37°C, is shown as a semilogarithmic plot in Figure
Effect of Olanzapine loading on polymer degradation.
Results from this study confirm that, for Olanzapine, substantial molecular weight reduction was noted at two distinct phases: after manufacture of the microspheres ( throughout the
As mentioned in the introduction (refer to Section
Rate constant and half-life for Placebo, Olanzapine, and Risperidone microspheres.
% drug loading | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
Parameter measured | 0 | 1 | 5 | 10 | 15 | 35 | ||||
Placebo | Risp. | Olanz. | Risp. | Olanz. | Risp. | Olanz. | Risp. | Olanz. | Risp. | |
|
0.003 | 0.148 | 0.047 | 0.141 | 0.077 | 0.184 | 0.083 | 0.359 | 0.080 | 0.306 |
|
213 | 4.68 | 14.6 | 4.91 | 8.97 | 3.77 | 8.35 | 1.93 | 8.63 | 2.267 |
|
— | 0.007 | — | 0.014 | — | 0.014 | — | 0.060 | — | 0.053 |
|
— | 102.3 | — | 48.1 | — | 49.5 | — | 11.5 | — | 13.2 |
In
Using the
In contrast to Risperidone, polymer degradation in Placebo and Olanzapine microspheres followed first order kinetics (Figure
Unlike Risperidone and Olanzapine, no lowering of polymer molecular weight was observed in the Placebo microsphere during manufacture; there was no difference between MWinitial and
Relationship between drug loading and molecular weight reduction in PLGA microspheres after encapsulation of Risperidone and Olanzapine.
A closer inspection of Figure
Interestingly, a linear relationship was also observed when the % change in molecular weight during manufacture was plotted against % drug loading (Figure
Effect of Risperidone and Olanzapine on % change in molecular weight during manufacture.
Similar to the plot described in Figure
To gain a better understanding of the extent to which Risperidone and Olanzapine degrade the 65 : 35 PLGA polymer when incubated in 0.02 M phosphate buffer saline pH 7.2 containing 0.05% Tween 80, at 37°C, the molecular weight at day 15 (
Effect of Risperidone and Olanzapine on % change in molecular weight after 15 days.
Extent of polymer degradation due to Risperidone and Olanzapine.
Since the molecular weight of the polymer in the microspheres (
Another parameter, that is, the extent of polymer degradation, was calculated to provide an insight to the extent which Risperidone and Olanzapine degrade the polymer. In essence, it is a comparison of the raw polymer molecular weight (MWinitial) against the molecular weight after the 15-day degradation study (
From Figure
In general, the findings presented in Sections
The rationale for implementing such a systematic formulation development approach is twofold and offers several advantages.
As per the formulation rationale presented above, drug loadings between 1 and 15% provided a critical insight about the drug : polymer ratio required for dosage form development. Additionally, since the commercially available Risperidone microspheres contain 38% drug load, a 35% loading of drug for Risperidone was also assessed [
To obtain a better understanding of the mechanism in which these atypical antipsychotics catalyze polymer hydrolysis, pH and temperature dependency of the degradation process was assessed. In comparing the effect of Risperidone and Olanzapine loadings on polymer degradation, the former has a lower effect on degradation. Hence, for the remainder of the experimental study, Risperidone was chosen as the preferred candidate to evaluate the mechanism by which basic nucleophiles cause degradation of the PLGA polymer and was selected as the model drug for additional studies.
To gain insight into the manner in which Risperidone influences polymer degradation, degradation studies were performed at acidic, neutral, and alkaline pH (pH 5.8, 7.2, and 9.6, resp., at 37°C in 0.02 M PBS containing 0.05% Tween 80) and compared against a control (Placebo microspheres).
Figure
Effect of pH on degradation of Risperidone microspheres at 37°C.
When compared to the Placebo microspheres, the presence of Risperidone impacted the reaction profile and increased the rate of degradation of the polymer at all the pH values studied. A biphasic degradation profile was observed with the Risperidone microspheres, similar to that presented in Figure
The effect of temperature on polymer degradation of Risperidone microspheres was investigated at 0 (Placebo), 1, and 5% loadings at 37°C and 55°C in 0.02 M PBS pH 7.2, and a comparison of the % decrease in polymer molecular weight after 2 days is shown (Figure
Effect of temperature on the % decrease in molecular weight of Risperidone microspheres after 2 days of incubation at 37 and 55°C.
It has been well established that the rate of a chemical reaction is influenced by temperature. With biodegradable polymers like PLGA or PLA, the relationship between temperature and polymer degradation has been explored by a few authors. For example, Buchholz examined the temperature dependency of neat polymer degradation at 37 and 80°C [
In the current study, since temperature effects on polymer degradation were expected to be profound, lower loadings of Risperidone microspheres (1 and 5%) were selected for evaluation and the results were compared with a control (Placebo microspheres). The results obtained from Figure
While the biodegradability, biocompatibility, and nonimmunogenicity of PLGA polymers have rendered them suitable for a variety of surgical and medical applications, including dosage forms, the degradative effects of basic and nucleophilic drugs Risperidone and Olanzapine on the polymer diminish their utility with respect to providing sustained levels of encapsulated therapeutic agents over extended intervals. Incorporation of such candidates into PLGA, even at low drug loadings, will greatly enhance polymer degradation and thereby influence formulation performance,
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research described in this paper was performed while the authors were affiliated with the University of Kentucky, Lexington, KY. The authors wish to thank Oakwood Labs, Oakwood, OH, and the Graduate School, University of Kentucky, Lexington, KY, for their financial support.