Rheological investigation of manufacturability and injectability of highly concentrated monoclonal antibody formulations

Highly concentrated protein therapeutics offer a convenient way for subcutaneous (sc) drug administration by the patient him-/herself or a healthcare professional. As the therapy e.g. with monoclonal antibodies requires quite high doses in the range of mg per kg body weight, the development of highly concentrated protein formulations is needed due to the limited injection volume, generally considered being 1 - 2 mL for sc administration. The development of highly concentrated formulations exceeding 50 - 100 mg/mL poses several challenges including chemical and physical stability (e.g. aggregation) as well as solution viscosity. Thereby, the increase in viscosity observed with higher protein concentration may cause severe limitations during product development as well as processing and drug administration. These limitations are defined by the flow rate/injection rate depending on the applied pressure which is needed during manufacture (fill-finish), in particular during filtration, and drug administration.
The focus of this work was to investigate the rheological behavior of protein solutions at high protein concentrations. The main objective was to obtain a profound understanding of two critical, hydrodynamic processes for highly concentrated protein solutions, which were drug administration and filtration, and to elucidate the role of viscosity with regard to potential limitations.
The current work provides a detailed overview on product characteristics of ten commercially available, highly concentrated protein therapeutics (Chapter 1). This technical overview summarizes formulation properties like viscosity and number of visible and sub-visible particles, physico-chemical properties like pH and osmolality as well as injection device characteristics, such as device dimensions. The analysis of marketed products revealed significant differences between the products. The current benchmark for maximum protein concentration and of viscosity was identified as a liquid formulation at a protein concentration of 200 mg/mL with a dynamic viscosity of 102 mPas (20? C). This product, which is provided in a pre-filled syringe, also exhibits the largest inner needle diameter of 25 G compared to other commercial products using 27 G needle for the injection device.
In the following (Chapter 2), advantages and limitations of different methods for viscosity determination of protein formulations are discussed. Moreover, a high-throughput method to measure viscosity was established. This method uses a capillary electrophoresis instrumentation without operation of the electrical field. The established method has the advantage of being automated offering the possibility for high-throughput by use of low sample amounts in the microliter range at the same time. (Allmendinger et al., J Pharm Biomed Anal, 99 (2014) 51-58)
Based on these studies, the present work investigated and characterized the subcutaneous drug administration process of highly concentrated protein formulations providing quantitative in vitro (Chapter 3) and in vivo data (G¨ottingen minipigs) of injection forces (Chapter 4).
Chapter 3 describes in detail the establishment of an in silico model to predict injection forces depending on syringe and needle dimensions, solution viscosity, and injection rate. Importantly, this model accounts for shear thinning behavior (non-Newtonian flow behavior) of highly concentrated protein olutions, which leads to lower effective injection forces than expected from current literature models. (Allmendinger et al., Eu J Pharm Biopharm, 87 (2014) 318-328)
To address the in vivo situation, Chapter 4 investigates and quantifies the contribution of the subcutaneous tissue backpressure and specifically reports the additional influence of body temperature on injection forces, which was found to compensate the tissue backpressure to some parts. Overall, an extended model, which addresses the injection force as a function of viscosity, volumetric flow/injection rate, needle/device dimensions, shear-thinning behavior, sc backpressure, and body temperature, was developed to predict injection forces representative for the in vivo situation. This knowledge is of key importance for the development of combination products (e.g. autoinjectors or pre-filled syringes) as a detailed understanding of injection forces depending on various parameters is required. It may be also supportive for the definition of limits during the evaluation, planning, and design phase during the development of injection devices. (Allmendinger et al., submitted to Pharm Research, 2014)
Besides drug administration, filtration was investigated as another critical hydrodynamic process for highly concentrated protein formulations, depending on formulation composition and filter material (Chapter 5). For both processes, filtration and drug administration, shear thinning behavior was found for some of the products depending on viscosity and protein concentration, shear rate, and formulation composition.
Within the present work it was shown, that the two investigated hydrodynamic processes, filtration and drug administration by injection, are two highly complex processes which are influenced by various factors. Thereby, the final limiting parameter for the injection process is given by the user capability of the patient population. However, the needle inner diameter was shown to have major influence on injection forces. It is related to injection forces by the power of four compared to other parameters like viscosity, injection rate, and contribution of sc backpressure being directly proportional. For the filtration process, the final limiting parameter may be discussed controversially. The study showed that the filtration pressure is mainly defined by the pore size distribution of the filter material, which was furthermore found to trigger the rheological behavior at high protein concentrations dependent on filtration rate. Moreover, literature data reported that the influence of filtration pressure on product quality might not be the limiting parameter during filtration. For the formulations previously tested, the shear stress exposure during manufacture was not considered important for final product quality, however only tested up to a protein concentration of 100 mg/mL. More important causes of aggregation were suggested to be the presence of air-bubbles, adsorption to solid surfaces, or contamination by particulates. Nevertheless, the stability of formulations showing pronounced shear-thinning behavior at high shear rates, which is most likely only the case for higher protein concentrations than previously tested, needs further experiments and has to be evaluated on a case-by-case basis dependent on product and process characteristics. (Allmendinger et al., submitted to J Pharm Sci, 2014)
With respect to viscosity, the current work has demonstrated for both processes, drug administration and filtration, that the potential limitation defined by the proportional increase in pressure based on Newtonian flow behavior was overestimated due to the presence of shear-thinning behavior which was shown for highly concentrated protein formulations.