Current Updates on In-vitro Drug Release Testing of Long-Acting Injectables

Thursday, March 24, 2022

Soumalya Chakraborty,a

Arvind K. Bansala#

aDepartment of Pharmaceutics, National Institute of Pharmaceutical Education and Research (NIPER)-S.A.S. Nagar, Sector 67, S.A.S. Nagar, Punjab-160062, India

#Corresponding author

Abstract 

Long acting injectables (LAIs) are drug products, intended for administration through the parenteral route and designed to release Active Pharmaceutical Ingredients (APIs) in a controlled manner for a prolonged period. In recent years a high number of NDA approval for LAIs by the U.S. FDA reflects a lucrative opportunity to pharmaceutical manufacturers for the development of new and generic LAIs. Understanding drug release is crucial for development of LAIs. Development of in vitro release testing specifications for LAIs is challenging and no regulatory guideline is available on this. This article captures the fundamentals and current scenario of development of in vitro drug release testing of LAIs with an aim to highlight the future areas of research.  

Keywords 

In vitro drug release, Long acting injectables, Generics, Accelerated in vitro release testing, In vitro-in vivo correlation  

Introduction 

LAIs are parenteral drug products, designed to release API in a controlled manner for a prolonged period.1  Release from LAIs can be from a formulation, implant or a device controlled. In this article focus will be restricted to formulation controlled LAIs only. 

The commercial significance of LAIs is demonstrated by the FDA’s approval of over 41 NDAs. But among them, only seven have generic versions available and at least nine reference listed LAIs are going to be eligible for generic approval within the upcoming five years.2 This landscape represents a significant opportunity to both new and generic product manufacturers.  

Technical constraints, requirement- of high investment, and lack of regulatory guidance impose challenges on the development of LAI generics. When developing LAIs, a good understanding of drug release kinetics in vivo is important for effective and robust performance of the drug product. Assessment of in vivo performance of each formulation batch is impractical due to constraints of time and expense.3  Development of an in vitro release testing method helps to indicate in vivo product performance and to avoid batch to batch variation. 

Development of an in vitro release testing method for LAIs is challenging because of the complex nature of drug release mechanisms, diversity in compendial and in-house release testing specifications and uniqueness of different formulation design approaches. In vitro drug release testing for LAIs could benefit from the availability of regulatory guidance and compendial information.4 

This article captures the fundamentals and current scenario of development of in vitro drug release testing of LAIs with an aim to highlight the areas for future research interest. 

Important Factors for Development of in vitro Release Testing Method for LAIs 

Drug stability 

The stability of both released and unreleased drugs is important during design of the release testing method for LAIs. The drug can be chemically degraded or physically denatured (for biologics) in the aqueous media over time and preventive measures should be taken.5,6 To maintain physiological significance, instead of adjustment in the release media, it is suggested to optimize the sampling pattern (total media removal) and analysis protocol (immediate analysis, adjustment of pH and storage temperature of sample). The drug can be degraded in the formulation before release.3  This issue should be addressed from the formulation design stage and appropriate adjustments should be made in the formulation. 

Sink condition 

Special care should be taken for maintenance of sink condition.7 A rule of thumb is to keep the dissolved drug concentration less than 10% of the saturation concentration at any given time.8  The sampling volume should be optimized based on the saturation concentration of drug to avoid any excursion from sink condition. When a drug has very low saturation concentration, total media removal during sampling is suggested.3 

Burst release 

The release profile of most of the LAIs begins with a burst release.9–11 Absence of any official definition of burst release creates variation in the description of the burst release phase. The latter is expressed both in terms of the release within a specific time point and the change in release kinetics.3  Characterizing the burst release phase is important and regulatory agencies should provide guidance. 

Release testing endpoint 

For LAIs 80% drug release of the claimed amount in label is accepted.12 The incomplete drug release should be entirely due to the formulation parameters, and not the degradation of the drug. Detailed analysis of the undissolved residue after the release testing should be performed and the residue should match quantitatively with the drug amount obtained after subtraction of the cumulative amount released from the theoretical drug loading. 

Biopredictivity 

The developed method should possess physiological relevance. Conditions that can be optimized are temperature, agitation (pattern and rate), sampling method, pH, buffer capacity of the media and time of the study.13 

Selection of dissolution vessel 

Vessel specification and its clarification (especially when USP compendia apparatus are not utilized) is another prime aspect. Release pattern may vary significantly depending on the geometry of the dissolution vessel.14 Apart from geometry, the material of construction should be properly optimized to avoid drug sorption, leaching and vessel deformation.3  

Methods for in vitro Release Testing of LAIs 

Based on the principle, methods for in vitro release testing can be categorized into three categories-(1) sample and separate (SS) method, (2) dialysis membrane (DM) method, and (3) continuous flow (CF) method (Figure 1).

In the SS method initially, micro- or nano-particles are suspended in release media and agitated at a particular temperature. Sampling is done at different time intervals, followed by the separation of particles by filtration and/or centrifugation. Finally the particles are resuspended with fresh media in the dissolution vessel. The limitations of SS method include particulate aggregation, large media volume, labor intensiveness and inaccuracies due to sample loss.15 

Figure 1. Schematic representation of (a) SS method,(b) DM method, (c) CF method

The DM method utilizes an appropriate dialysis membrane with a specific molecular-weight cut-off (MWCO) to physically separate the released drug molecules from microparticles. Drug release is usually assessed with samples taken from the external solution outside the DM over time. The DM method suffers from several disadvantages, including underestimation of the initial drug levels due to slow equilibration with the outer medium, microparticle aggregation, binding of drug with DM and violation of sink condition inside dialysis sac.16 

The CF method utilizes a system equipped with a flow-through cell, a pump and medium reservoir. The micro- or nano-particle sample is put into a small volume cell, the release medium flows through the cell. The released drugs pass through the filter in the flow-through cell.17 This method imposes hurdles for very long release period due to clogging of the filter by particles, causing failure of the filter and generation of back pressure.3,16 

Due to the lack of standard in vitro release testing method for LAIs various methods including USP apparatus for oral route and different customized methods have been utilized.3  The use of USP apparatus 2,18,19 USP apparatus 420 and orbital agitation method21–23 have been reported. USP apparatus 4 is an excellent alternative for short-term studies, with a high robustness and discriminatory power. However owing to higher cost, it may not be available in many laboratories. As already mentioned, it also produces several challenges for long term use. In such instances, USP apparatus 4 and orbital agitation methods may be more suitable option. The optimization of method in USP apparatus 2 is easier than orbital agitation methods, as the standard set up becomes available for the former. The testing conditions of orbital agitation methods, such as volume, agitation rates, and container type, may differ from case to case basis, generating the requirement of optimization for each method development attempt.24  

Accelerated in vitro Release Testing for LAIs 

The real time in vitro release resting for LAIs may require periods of week, months or even years, leading to increased time of batch release, thus reducing the effective product shelf life.15 Sometimes LAIs require preservatives to perform real-time release testing at 37 °C. This may generate questions about the compatibility of release test device components such as tubings and membranes.3  

Table 1. Examples of diff erent parameters and conditions for accelerated release testing for LAIs

Accelerated in vitro release methods have received considerable attention to shorten the time for drug release study. Acceleration can be achieved by altering different parameters, such as- temperature, pH, test medium composition, surfactants, and agitation rate (Table1).17 

The extreme conditions may alter the drug release mechanism or stability.28 Therefore knowledge about the impact of acceleration conditions on drug release mechanism and stability is crucial. 

Drug release from “real-time” and accelerated tests should follow the identical release mechanism.15 The specific correlation should be established at three-time points- one initial, one intermediate and one-time point exceeding 80% of the cumulative drug released. Accelerated test methods should evaluate any initial burst release. It may be difficult if the burst release phase is very fast. In such cases, a simultaneous real-time release study is suggested.16 

In vitro-in vivo Correlation (IVIVC) for LAIs 

According to the FDA guidance IVIVC is defined as “a predictive mathematical model describing the relationship between an in vitro property of an extended-release dosage form and a relevant in vivo response”.29 FDA guidance states about level A, B, C, D and multiple level C correlation. Level A indicates the highest correlation and biowaiver can be applied only for level A.29 Some of the level A IVIVCs for LAIs is shown in Table 2.

Table 2. Examples of level A IVIVCs for LAIs

Somayaji et al proposed a new level A IVIVC to accelerate the development and approval of generic LAIs. A conventional Level A Type IVIVC compares the in vitro and in vivo outcomes of the same generic formulation, the newly proposed method directly correlates the in vitro outcomes of candidate generic formulations with the clinical absorption characteristics of a reference listed drug. However, the scope of this method is limited to shortlisting of the specific generic LAIs for clinical BE evaluation, and not to analyze drug performances (i.e., drug safety and effectiveness).34 

The absence of regulatory guideline imposes significant challenges in developing IVIVC for LAIs.35 Generally, IVIVC guidance for oral drug products is applied to describe the same for LAIs. However, FDA-funded collaborative research for IVIVC development for LAIs with different laboratories is ongoing. Up until 2015 the FDA funded a minimum of three research projects on IVIVC,36 that have unveiled different critical aspects, especially the effect of small manufacturing changes on IVIVC establishment for LAIs.37 

Current FDA Initiatives on in vitro Release Testing of LAIs 

Since 2012 the FDA has funded more than 16 research projects on different aspects of LAIs,36 especially the latest in vitro release testing. technologies and the potential challenges associated with it in context of different formulations. This research has helped the FDA in identifying promising technologies, current challenges and limitations in in vitro release testing method development, and to have a better understanding on the critical quality attributes of complex products.38 A strong research initiative has been implemented involving different physiologically based pharmacokinetic (PBPK) modeling software in establishment of IVIVC and virtual bioequivalence for LAIs.39,40 The findings are publically shared through workshops, presentations, and academic publications. USP has published a draft chapter on “In vitro release test methods for parenteral drug preparations”.41 The increased effort of regulatory bodies towards developing knowledge base on different critical aspects of LAI product development is promising and provides a strong understanding to industry for LAI product development.  

Conclusion 

In the last decade, LAIs have shown promise in commercialization of both new and generic product manufacturers. In vitro release testing is important for successful product development. The absence of official guidelines or requirements about in vitro release testing for LAIs leads to inter-lab variability. Also IVIVC for LAIs is established based on the principles involved in IVIVC guidance for oral products. Most of the developmental work regarding LAIs has been done on microsphere and multi-vesicular liposome-based formulation. Research work on other approaches should be accelerated to unveil their role in release specification development.  

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Author Biographies 

Arvind K. Bansal is currently Professor and Head of the Department of Pharmaceutics at the National Institute of Pharmaceutical Education and Research (NIPER)-S.A.S. Nagar. He received his PhD in 1993 from the University of Delhi and has served as a research scientist in the major Indian pharmaceutical industries. He has more than 25 years of experience in the fi elds of amorphous systems, particle engineering, compaction physics, lyophilization and formulation development of nanocrystal and cocrystal-based products. 

Soumalya Chakraborty is pursuing his Ph.D. in the Department of Pharmaceutics at the NIPER-S.A.S. Nagar under the supervision of Prof. Arvind K. Bansal. He is working on understanding the release mechanism of amorphous solid dispersion based drug-products. He earned his M.S. (Pharm.) from the NIPER-S.A.S. Nagar. His research focuses on the amorphous solid dispersion, solid-state properties of pharmaceutical materials, formulation development for oral and parenteral dosage forms, and transfer of technologies from lab scale to pilot scale.

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