Xinchang Shi

Background/Objectives: Recombinant adeno-associated virus (rAAV) has become a leading vector in gene therapy. However, manufacturing limitations may result in replication-competent AAV (rcAAV) contamination of clinical rAAV products, posing safety risks. Rigorous testing is therefore essential, and the use of accurately calibrated rcAAV reference standard materials is critical for ensuring assay stability and reliability. A disadvantage of the widely used Tissue Culture Infectious Dose 50 (TCID₅₀) assay is its high variability. This study introduces an optimized TCID₅₀ assay for the precise quantification of infectious rcAAV particles. Methods: We developed a TCID₅₀ assay tailored to rep2-based rcAAV, optimizing key aspects such as viral infection conditions, qPCR reaction systems, and standard curve preparation. We employed an innovative strategy to prepare the standard curve using serial dilutions of rcAAV in cell lysate, ensuring alignment with the test sample matrices. Results: The rcAAV-derived standard curve demonstrated exceptional linearity (R² > 0.99), sensitivity (LOQ ≈ 38 copies), and reproducibility, enabling robust endpoint qPCR analysis. The optimized assay significantly improved the precision of the TCID₅₀ assay, as an inter-assay coefficient of variation (CV) of 11.4% was achieved. Conclusions: This refined TCID₅₀ assay is a reliable method for calibrating infectious titers of rcAAV reference standard materials, thereby enabling the standardization of rcAAV testing.

Human granulocyte colony-stimulating factor (hG-CSF) is primarily used to treat neutropenia induced by cancer chemotherapy and bone marrow transplantation. The current identification test for hG-CSF relies on Western blot (WB), a labor-intensive and technically demanding method. This study aimed to screen and prepare an anti-hG-CSF nanobody to identify and quantify hG-CSF, with the ultimate goal of developing colloidal gold-labeled nanobody test strips for rapid identification. An alpaca was immunized with hG-CSF, and the VHH gene sequence encoding the anti-hG-CSF nanobody was obtained through sequencing following phage display library construction and multiple rounds of biopanning. The nanobody C68, obtained from screening, was expressed by E. coli, and its physicochemical properties such as molecular weight, isoelectric point, and affinity were characterized after purification. WB analysis demonstrated excellent performance of the nanobody in identification tests in terms of specificity, limit of detection (LOD), applicability with products from various manufacturers, and thermal stability. Additionally, we established an ELISA method for hG-CSF quantification utilizing the nanobody C68 and conducted methodological validation. Finally, colloidal gold-based test strips were constructed using the nanobody C68, with a LOD of 30 μg/mL, achieving rapid identification for hG-CSF. This study represents a novel application of nanobodies in pharmaceutical testing and offers valuable insights for developing identification tests for other recombinant protein drugs.

Currently, adeno-associated virus (AAV) is one of the primary gene delivery vectors in gene therapy, facilitating long-term in vivo gene expression. Despite being imperative, it is incredibly challenging to precisely assess AAV particle distribution according to the sedimentation coefficient and identify impurities related to capsid structures. This study performed the systematic methodological validation of quantifying the AAV empty and full capsid ratio. This includes specificity, accuracy, precision, linearity, and parameter variables involving the sedimentation velocity analytical ultracentrifugation (SV-AUC) method. Specifically, SV-AUC differentiated among the empty, partial, full, and high sedimentation coefficient substance (HSCS) AAV particles while evaluating their sedimentation heterogeneity. The intermediate precision analysis of HE (high percentage of empty capsid) and HF (high percentage of full capsid) samples revealed that the specific species percentage, such as empty or full, was more significant than 50%. Moreover, the relative standard deviation (RSD) could be within 5%. Even for empty or partially less than 15%, the RSD could be within 10%. The accuracy recovery rates of empty capsid were between 103.9% and 108.7% across three different mixtures. When the measured percentage of specific species was more significant than 14%, the recovery rate was between 77.9% and 106.6%. Linearity analysis revealed an excellent linear correlation between the empty, partial, and full in the HE samples. The AAV samples with as low as 7.4 × 10 ¹¹ cp/mL AAV could be accurately quantified with SV-AUC. The parameter variable analyses revealed that variations in cell alignment significantly affected the overall results. Still, the detection wavelength of 235 nm slightly influenced the empty, partial, and full percentages. Minor detection wavelength changes showed no impact on the sedimentation coefficient of these species. However, the temperature affected the measured sedimentation coefficient. These results validated the SV-AUC method to quantify AAV. This study provides solutions to AAV empty and full capsid ratio quantification challenges and the subsequent basis for calibrating the AAV empty capsid system suitability substance. Because of the AAV structure and potential variability complexity in detection, we jointly calibrated empty capsid system suitability substance with three laboratories to accurately detect the quantitative AAV empty and full capsid ratio. The empty capsid system suitability substance could be used as an external reference to measure the performance of the instrument. The results could be compared with multiple QC (quality control) laboratories based on the AAV vector and calibration accuracy. This is crucial for AUC to be used for QC release and promote gene therapy research worldwide.

Recombinant human erythropoietin (rhEPO) is one of the most important biopharmaceuticals worldwide, with global sales expected to reach US$11.9 billion in 2020.

The long-acting growth hormone (LAGH) is a promising alternative biopharmaceutical to treat growth hormone (GH) deficiency in children, and it was developed using a variety of technologies by several pharmaceutical companies. Most LAGH preparations, such as Fc fusion protein, are currently undergoing preclinical study and clinical trials. Accurate determination of bioactivity is critical for the efficacy of quality control systems of LAGH. The current in vivo rat weight gain assays used to determine the bioactivity of recombinant human GH (rhGH) in pharmacopoeias are time-consuming, expensive, and imprecise, and there are no recommended bioassays for LAGH bioactivity in pharmacopoeias. Therefore, we developed a cell-based bioassay for bioactivity determination of therapeutic long-acting Fc-fusion recombinant human growth hormone (rhGH-Fc) based on the luciferase reporter gene system, which is involved in the full-length human GH receptor (hGHR) and the SG (SIE and GAS) response element. The established bioassay was comprehensively validated according to the International Council for Harmonization (ICH) Q2 (R1) guidelines and the Chinese Pharmacopoeia, and is highly precise, time-saving, simple, and robust. The validated bioassay could be qualified for bioactivity determination during the research, development, and manufacture of rhGH-Fc, and other LAGH formulations.

The therapeutic recombinant human keratinocyte growth factor 1 (rhKGF-1) was approved by the FDA for oral mucositis resulting from hematopoietic stem cell transplantation for hematological malignancies in 2004. However, no recommended bioassay for rhKGF-1 bioactivity has been recorded in the U.S. Pharmacopoeia. In this study, we developed an rhKGF-1-dependent bioassay for determining rhKGF-1 bioactivity based on HEK293 and HaCat cell lines that stably expressed the luciferase reporter driven by the serum response element (SRE) and human fibroblast growth factor receptor (FGFR2) IIIb. A good responsiveness to rhKGF-1 and rhKGF-2 shared by target HEK293/HaCat cell lines was demonstrated. Our stringent validation was completely focused on specificity, linearity, accuracy, precision, and robustness according to the International Council for Harmonization (ICH) Q2 (R1) guidelines, AAPS/FDA Bioanalytical Workshop and the Chinese Pharmacopoeia. We confirmed the reliability of the method in determining rhKGF bioactivity. The validated method is highly timesaving, sensitive, and simple, and is especially valuable for providing information for quality control during the manufacture, research, and development of therapeutic rhKGF.