Emerging technologies for quality control of cell-based, advanced therapy medicinal products.

Advanced therapy medicinal products (ATMP) are complex medicines based on gene therapy, somatic cell therapy, and tissue engineering. These products are rapidly arising as novel and promising therapies for a wide range of different clinical applications. The process for the development of well-established ATMPs is challenging. Many issues must be considered from raw material, manufacturing, safety, and pricing to assure the quality of ATMPs and their implementation as innovative therapeutic tools. Among ATMPs, cell-based ATMPs are drugs altogether. As for standard drugs, technologies for quality control, and non-invasive isolation and production of cell-based ATMPs are then needed to ensure their rapidly expanding applications and ameliorate safety and standardization of cell production. In this review, emerging approaches and technologies for quality control of innovative cell-based ATMPs are described. Among new techniques, microfluid-based systems show advantages related to their miniaturization, easy implementation in analytical process and automation which allow for the standardization of the final product.

Impact of Porcine Pancreas Decellularization Conditions on the Quality of Obtained dECM.

Due to the limited number of organ donors, 3D printing of organs is a promising technique. Tissue engineering is increasingly using xenogeneic material for this purpose. This study was aimed at assessing the safety of decellularized porcine pancreas, together with the analysis of the risk of an undesirable immune response. We tested eight variants of the decellularization process. We determined the following impacts: rinsing agents (PBS/NH3·H2O), temperature conditions (4 °C/24 °C), and the grinding method of native material (ground/cut). To assess the quality of the extracellular matrix after the completed decellularization process, analyses of the following were performed: DNA concentration, fat content, microscopic evaluation, proteolysis, material cytotoxicity, and most importantly, the Triton X-100 content. Our analyses showed that we obtained a product with an extremely low detergent content with negligible residual DNA content. The obtained results confirmed the performed histological and immuno-fluorescence staining. Moreover, the TEM microscopic analysis proved that the correct collagen structure was preserved after the decellularization process. Based on the obtained results, we chose the most favorable variant in terms of quality and biology. The method we chose is an effective and safe method that gives a chance for the development of transplant and regenerative medicine.

GMP compliant isolation of mucosal epithelial cells and fibroblasts from biopsy samples for clinical tissue engineering.

Engineered epithelial cell sheets for clinical replacement of non-functional upper aerodigestive tract mucosa are regulated as medicinal products and should be manufactured to the standards of good manufacturing practice (GMP). The current gold standard for growth of epithelial cells for research utilises growth arrested murine 3T3 J2 feeder layers, which are not available for use as a GMP compliant raw material. Using porcine mucosal tissue, we demonstrate a new method for obtaining and growing non-keratinised squamous epithelial cells and fibroblast cells from a single biopsy, replacing the 3T3 J2 with a growth arrested primary fibroblast feeder layer and using pooled Human Platelet lysate (HPL) as the media serum supplement to replace foetal bovine serum (FBS). The initial isolation of the cells was semi-automated using an Octodissociator and the resultant cell suspension cryopreservation for future use. When compared to the gold standard of 3T3 J2 and FBS containing medium there was no reduction in growth, viability, stem cell population or ability to differentiate to mature epithelial cells. Furthermore, this method was replicated with Human buccal tissue, providing cells of sufficient quality and number to create a tissue engineered sheet.

Recent Progress Toward Clinical Translation of Tissue-Engineered Heart Valves.

Surgical replacement remains the primary option to treat the rapidly growing number of patients with severe valvular heart disease. Although current valve replacements-mechanical, bioprosthetic, and cryopreserved homograft valves-enhance survival and quality of life for many patients, the ideal prosthetic heart valve that is abundantly available, immunocompatible, and capable of growth, self-repair, and life-long performance has yet to be developed. These features are essential for pediatric patients with congenital defects, children and young adult patients with rheumatic fever, and active adult patients with valve disease. Heart valve tissue engineering promises to address these needs by providing living valve replacements that function similarly to their native counterparts. This is best evidenced by the long-term clinical success of decellularised pulmonary and aortic homografts, but the supply of homografts cannot meet the demand for replacement valves. A more abundant and consistent source of replacement valves may come from cellularised valves grown in vitro or acellular off-the-shelf biomaterial/tissue constructs that recellularise in situ, but neither tissue engineering approach has yet achieved long-term success in preclinical testing. Beyond the technical challenges, heart valve tissue engineering faces logistical, economic, and regulatory challenges. In this review, we summarise recent progress in heart valve tissue engineering, highlight important outcomes from preclinical and clinical testing, and discuss challenges and future directions toward clinical translation.

Standardization of Microcomputed Tomography for Tracheal Tissue Engineering Analysis.

Tracheal tissue engineering has become an active area of interest among clinical and scientific communities; however, methods to evaluate success of in vivo tissue-engineered solutions remain primarily qualitative. These evaluation methods have generally relied on the use of photographs to qualitatively demonstrate tracheal patency, endoscopy to image healing over time, and histology to determine the quality of the regenerated extracellular matrix. Although those generally qualitative methods are valuable, they alone may be insufficient. Therefore, to quantitatively assess tracheal regeneration, we recommend the inclusion of microcomputed tomography (µCT) to quantify tracheal patency as a standard outcome analysis. To establish a standard of practice for quantitative µCT assessment for tracheal tissue engineering, we recommend selecting a constant length to quantify airway volume. Dividing airway volumes by a constant length provides an average cross-sectional area for comparing groups. We caution against selecting a length that is unjustifiably large, which may result in artificially inflating the average cross-sectional area and thereby diminishing the ability to detect actual differences between a test group and a healthy control. Therefore, we recommend selecting a length for µCT assessment that corresponds to the length of the defect region. We further recommend quantifying the minimum cross-sectional area, which does not depend on the length, but has functional implications for breathing. We present empirical data to elucidate the rationale for these recommendations. These empirical data may at first glance appear as expected and unsurprising. However, these standard methods for performing µCT and presentation of results do not yet exist in the literature, and are necessary to improve reporting within the field. Quantitative analyses will better enable comparisons between future publications within the tracheal tissue engineering community and empower a more rigorous assessment of results. Impact statement The current study argues for the standardization of microcomputed tomography (µCT) as a quantitative method for evaluating tracheal tissue-engineered solutions in vivo or ex vivo. The field of tracheal tissue engineering has generally relied on the use of qualitative methods for determining tracheal patency. A standardized quantitative evaluation method currently does not exist. The standardization of µCT for evaluation of in vivo studies would enable a more robust characterization and allow comparisons between groups within the field. The impact of standardized methods within the tracheal tissue engineering field as presented in the current study would greatly improve the quality of published work.

Evaluation of a cell-based osteogenic formulation compliant with good manufacturing practice for use in tissue engineering.

Proper bony tissue regeneration requires mechanical stabilization, an osteogenic biological activity and appropriate scaffolds. The latter two elements can be combined in a hydrogel format for effective delivery, so it can readily adapt to the architecture of the defect. We evaluated a Good Manufacturing Practice-compliant formulation composed of bone marrow-derived mesenchymal stromal cells in combination with bone particles (Ø = 0.25 to 1 µm) and fibrin, which can be readily translated into the clinical setting for the treatment of bone defects, as an alternative to bone tissue autografts. Remarkably, cells survived with unaltered phenotype (CD73+, CD90+, CD105+, CD31-, CD45-) and retained their osteogenic capacity up to 48 h after being combined with hydrogel and bone particles, thus demonstrating the stability of their identity and potency. Moreover, in a subchronic toxicity in vivo study, no toxicity was observed upon subcutaneous administration in athymic mice and signs of osteogenesis and vascularization were detected 2 months after administration. The preclinical data gathered in the present work, in compliance with current quality and regulatory requirements, demonstrated the feasibility of formulating an osteogenic cell-based tissue engineering product with a defined profile including identity, purity and potency (in vitro and in vivo), and the stability of these attributes, which complements the preclinical package required prior to move towards its use of prior to its clinical use.

Extrusion-Based Bioprinting: Current Standards and Relevancy for Human-Sized Tissue Fabrication.

The field of bioengineering has long pursued the goal of fabricating large-scale tissue constructs for use both in vitro and in vivo. Recent technological advances have indicated that bioprinting will be a key technique in manufacturing these specimens. This chapter aims to provide an overview of what has been achieved to date through the use of microextrusion bioprinters and what major challenges still impede progress. Microextrusion printer configurations will be addressed along with critical design characteristics including nozzle specifications and bioink modifications. Significant challenges within the field with regard to achieving long-term cell viability and vascularization, and current research that shows promise in mitigating these challenges in the near future are discussed. While microextrusion is a broad field with many applications, this chapter aims to provide an overview of the field with a focus on its applications toward human-sized tissue constructs.

Manufacturing considerations for producing and assessing decellularized extracellular matrix hydrogels.

Although several decellularized extracellular matrix (ECM) sheets or patches have been commercialized for use in the clinic, only one injectable decellularized ECM hydrogel, a decellularized myocardial matrix, has reached clinical trials. Consequently, very little information is available for established manufacturing standards or assessments of these materials. Here we present detailed methodology for investigating three parameters related to manufacturing optimization for a porcine derived skeletal muscle ECM hydrogel - animal-to-animal variability, bioburden reduction, and harvesting conditions. Results from characterization assays, including residual dsDNA content and sulfated glycosaminoglycan content, did not yield noteworthy differences amongst individual animals or following the addition of a bioburden reducing agent. However, the tissue collected under different harvesting conditions contained varying amounts of fat, and the protein compositions of the decellularized products differed, which could ultimately impact subsequent efficacy in vitro or in vivo. As decellularized ECM hydrogels continue to be evaluated for various applications, the differences between laboratory-scale and manufacturing-scale material batches should be thoroughly considered to avoid costly and timely optimization during scale-up.

[Effect of icariin/attapulgite/collagen type â /polycaprolactone composite scaffold in repair of rabbit tibia defect].

OBJECTIVE: To investigate the effect of icarin/attapulgite/collagen type Ⅰ/polycaprolactone (ICA/ATP/Col Ⅰ/PCL) composite scaffold in repair of rabbit tibia defect. METHODS: The ICA/20%ATP/Col Ⅰ/PCL (scaffold 1), ICA/30%ATP/Col Ⅰ/PCL (scaffold 2), 20%ATP/Col Ⅰ/PCL (scaffold 3), and 30%ATP/Col Ⅰ/PCL (scaffold 4) composite scaffolds were constructed by solution casting-particle filtration method. The structure characteristics of the scaffold 2 before and after cross-linking were observed by scanning electron microscopy, and the surface contact angles of the scaffold 2 and the scaffold 4 were used to evaluate the water absorption performance of the material. The in vitro degradation test was used to evaluate the sustained-release effect of the scaffold 2. Thirty male Japanese white rabbits, weighing (2.0±0.1) kg, were randomly divided into groups A, B, C, D, and E, 6 in each group. After making a 1 cm- diameter bilateral tibial defects model, group A was the defect control group without any material implanted. Groups B, C, D, and E were implanted with scaffolds 3, 4, 1, and 2 at the defect sites, respectively. At 4, 8, and 12 weeks after operation, the repairing effects of 4 scaffolds were observed by gross observation, histological observation of HE and Masson staining, and immunohistochemical staining of osteogenic specific transcription factor (runt-related transcription factor 2, RUNX2), osteogenic related transcription factor [Osterix (OSX), Col Ⅰ, osteopontin (OPN)]. RESULTS: Scanning electron microscopy observation showed that the scaffolds were all porous. The structure of the material was loose before and after cross-linking. The surface contact angle showed that the scaffold was hydrophobic, and the scaffold 2 was more hydrophobic than scaffold 4. The sustained-release effect in vitro showed that the drug could be released in a micro and long-term manner. In the animal implantation experiment, the gross observation showed that the defects were significantly smaller in groups D and E than in groups A, B, and C at 4 and 12 weeks after operation. HE and Masson staining showed that the defect of group A was full of connective tissue at 4 weeks after operation, a large number of fibers were seen in groups B and C, and the new bone formation was observed in groups D and E. The increase of new bone was observed in each group at 8 weeks after operation. The defect of group A was still dominated by connective tissue at 12 weeks after operation, and a small amount of new bone tissue was observed in groups B and C, and a large number of new bone tissue was observed in groups D and E, especially in group E, and most of the materials degraded. Immunohistochemical staining showed that the expressions of RUNX2 and OSX in the new tissues of groups D and E were significantly higher than those of the other groups at 4 weeks after operation. The expression of RUNX2 decreased at 8 and 12 weeks after operation. After 8 weeks and 12 weeks, the expressions of Col Ⅰand OPN increased than in 4 weeks. And the expressions of Col Ⅰ and OPN in the new tissues of groups D and E were significantly more than those of the other groups. CONCLUSION: ICA/ATP/Col I/PCL composite scaffolds have good porosity and biocompatibility, can promote bone formation, and have good bone regeneration and repair effect.

3D printing of musculoskeletal tissues: impact on safety and health at work.

Additive manufacturing (commonly referred to as 3D printing) created an attractive approach for regenerative medicine research in musculoskeletal tissue engineering. Given the high number of fabrication technologies available, characterized by different working and physical principles, there are several related risks that need to be managed to protect operators. Recently, an increasing number of studies demonstrated that several types of 3D printers are emitters of ultrafine particles and volatile organic compounds whose harmful effects through inhalation, ingestion and skin uptake are known. Confirmation of danger of these products is not yet final, but this provides a basis to adopt preventive measures in agreement with the precautionary principle. The purpose of this investigation was to provide a useful tool to the researcher for managing the risks related to the use of different kinds of three-dimensional printers (3D printers) in the lab, especiallyconcerning orthopedic applications, and to define appropriate control measures. Particular attention was given to new emerging risks and to developing response strategies for a comprehensive coverage of the health and safety of operators.

Cartilage mechanical tests: Evolution of current standards for cartilage repair and tissue engineering. A literature review.

BACKGROUND: Repair procedures and tissue engineering are solutions available in the clinical practice for the treatment of damaged articular cartilage. Regulatory bodies defined the requirements that any products, intended to regenerate cartilage, should have to be applied. In order to verify these requirements, the Food and Drug Administration (FDA, USA) and the International Standard Organization (ISO) indicated some Standard tests, which allow evaluating, in a reproducible way, the performances of scaffolds/treatments for cartilage tissue regeneration. METHODS: A review of the literature about cartilage mechanical characterization found 394 studies, from 1970 to date. They were classified by material (simulated/animal/human cartilage) and method (theoretical/applied; static/dynamic; standard/non-standard study), and analyzed by nation and year of publication. FINDINGS: While Standard methods for cartilage mechanical characterization still refer to studies developed in the eighties, expertise and interest on cartilage mechanics research are evolving continuously and internationally, with studies both in vitro - on human and animal tissues - and in silico, dealing with tissue function and modelling, using static and dynamic loading conditions. INTERPRETATION: there is a consensus on the importance of mechanical characterization that should be considered to evaluate cartilage treatments. Still, relative Standards need to be updated to describe advanced constructs and procedures for cartilage regeneration in a more exhaustive way. The use of the more complex, fibre-reinforced biphasic model, instead of the standard simple biphasic model, to describe cartilage response to loading, and the standardisation of dynamic tests can represent a first step in this direction.

Evaluation of Proliferation and Osteogenic Differentiation of Human Umbilical Cord-Derived Mesenchymal Stem Cells in Porous Scaffolds.

INTRODUCTION: Human umbilical cord-derived mesenchymal stem cells (UCMSCs) are multiple potential stem cells that can differentiate into various kinds of functional cells, including adipocytes, osteoblasts, and chondroblasts. Thus, UCMSCs have recently been used in both stem cell therapy and tissue engineering applications to produce various functional tissues. This study aimed to evaluate the proliferation and differentiation of UCMSCs on porous scaffolds. METHODS: UCMSCs were established in a previous study and kept in liquid nitrogen. They were thawed and expanded in vitro to yield enough cells for further experiments. The cells were characterized as having MSC phenotype. They were seeded onto culture medium-treated porous scaffolds or on non-treated porous scaffolds at different densities of UCMSCs (105, 2.1 × 105, and 5 × 105 cells/0.005 g scaffold). The existence of UCMSCs on the scaffold was evaluated by nucleic staining using Hoechst 33342 dye, while cell proliferation on the scaffold was determined by MTT assay. Osteogenic differentiation was evaluated by changes in cellular morphology, accumulation of extracellular calcium, and expression of osteoblast-specific genes (including runx2, osteopontin (OPN), and osteocalcin (OCN)). RESULTS: The data showed that UCMSCs could attach, proliferate, and differentiate on both treated and non-treated scaffolds but were better on the treated scaffold. At a cell density of 105 cells/0.005 g scaffold, the adherent and proliferative abilities of UCMSCs were higher than that of the other densities after 14 days of culture (p < 0.05). Adherent UCMSCs on the scaffold could be induced into osteoblasts in the osteogenic medium after 21 days of induction. These cells accumulated calcium in the extracellular matrix that was positive with Alizarin Red staining. They also expressed some genes related to osteoblasts, including runx2, OPN, and OCN. CONCLUSION: UCMSCs could adhere, proliferate, and differentiate into osteoblasts on porous scaffolds. Therefore, porous scaffolds (such as Variotis) may be suitable scaffolds for producing bone tissue in combination with UCMSCs.

Current state of U.S. Food and Drug Administration regulation for cellular and gene therapy products: potential cures on the horizon.

Cellular & Gene Therapies (CGTs) are complex products, which have been key foci of the International Society for Cell & Gene Therapy (ISCT). For this ISCT North American Legal & Regulatory Affairs Committee review publication, CGTs include but are not limited to somatic cell-based therapies, pluripotent cell-derived cell-based therapies, gene- or non-gene-modified or gene edited versions of these cell-based therapies, in vivo gene therapies, organ/tissue engineered products, and relevant combination products. These products are regulated by the Food and Drug Administration (FDA) in the United States. This publication reviews selected laws, regulations, guidance, definitions, processes, types of meetings and submissions, and other key factors that the FDA follows and implements to regulate and support development of these types of products. These factors may be considered in order to help current and potential product developers/sponsors/applicants navigate through FDA regulatory pathways. We also review expedited programs including types of Designations available at the FDA, and their specific eligibility criteria. We include FDA and other stakeholder resources to consider regarding CGT regulation, to help prepare for CGT development and subsequent FDA approval.

Improving the biocompatibility of biomaterial constructs and constructs delivering cells for the pelvic floor.

PURPOSE OF REVIEW: Interactions between biomaterials and biomaterial-delivering cells and the host tissues are complexly affected by the material itself, the ultrastructure of the overall construct and cells and other bioactive factors involved. The aim of this review is to review the current understanding on the definitions of biocompatibility and current advances in improving biocompatability of tissue-engineered constructs. RECENT FINDINGS: Some synthetic materials are associated with more foreign body reactions compared with natural materials; however, they allow fabrication of materials with a great diversity of physical and mechanical properties. Material design strategies can be tailored to mimic the natural extracellular matrix topography. There are also advancements in the pharmacological functionalization of materials with improved angiogenic potential that can lead to better tissue response. Stem cells are also used to improve the tissue response of tissue-engineered materials; however, the recent regulations on regenerative medicine products necessitate significant regulatory approval processes for these. SUMMARY: The biggest challenge faced in translation of tissue-engineered constructs into clinical practice relates to their engraftment and poor tissue integration into the challenging wound bed of the pelvic floor. Biocompatibility of tissue engineered constructs can theoretically be improved by the incorporation of bioactive agents, such as vitamins C or oestradiol.

A Standardized and Quality-Controllable Protocol of Constructing Individual Tissue-Engineered Grafts Applicable to Treating Large Bone Defects.

Patient-specific individual tissue-engineered bones (iTEBs) have been recognized as a promising strategy for treating large bone defects. However, current construction protocols of iTEBs vary between lots and lack standardization and quality control, hampering further research and application. This study was aimed to detail a standardized constructing protocol for iTEBs, which can be used for both clinical and experimental purposes. The procedure was designed and described as follows: scaffold preparation, cell isolation and culture, and fabrication of iTEBs. Manipulation and caution points in each section were detailed. A series of scales on the quality control and safety monitoring was developed. The effectiveness and safety of iTEBs were evaluated. Eventually, the preparing portion, from cell culture to scaffold treatment, usually required 21 days. Generally, the fabrication section took 5 days. The main advantage of this protocol was that each step was standardized and quality controlling and safety monitoring were performed throughout the process to ensure the homogeneity, reliability, and safety. The resulting iTEBs were effective and applicable to both clinical and experimental purposes. Thus, we have established a refined and standardized protocol detailing the construction process of patient-specific iTEBs that comply with strict quality control and safety criteria. This protocol is relatively easy for graduate students or staff working in the field of bone tissue engineering to implement.

ESC Working Group on Cellular Biology of the Heart: position paper for Cardiovascular Research: tissue engineering strategies combined with cell therapies for cardiac repair in ischaemic heart disease and heart failure.

Morbidity and mortality from ischaemic heart disease (IHD) and heart failure (HF) remain significant in Europe and are increasing worldwide. Patients with IHD or HF might benefit from novel therapeutic strategies, such as cell-based therapies. We recently discussed the therapeutic potential of cell-based therapies and provided recommendations on how to improve the therapeutic translation of these novel strategies for effective cardiac regeneration and repair. Despite major advances in optimizing these strategies with respect to cell source and delivery method, the clinical outcome of cell-based therapy remains unsatisfactory. Major obstacles are the low engraftment and survival rate of transplanted cells in the harmful microenvironment of the host tissue, and the paucity or even lack of endogenous cells with repair capacity. Therefore, new ways of delivering cells and their derivatives are required in order to empower cell-based cardiac repair and regeneration in patients with IHD or HF. Strategies using tissue engineering (TE) combine cells with matrix materials to enhance cell retention or cell delivery in the transplanted area, and have recently received much attention for this purpose. Here, we summarize knowledge on novel approaches emerging from the TE scenario. In particular, we will discuss how combinations of cell/bio-materials (e.g. hydrogels, cell sheets, prefabricated matrices, microspheres, and injectable matrices) combinations might enhance cell retention or cell delivery in the transplantation areas, thereby increase the success rate of cell therapies for IHD and HF. We will not focus on the use of classical engineering approaches, employing fully synthetic materials, because of their unsatisfactory material properties which render them not clinically applicable. The overall aim of this Position Paper from the ESC Working Group Cellular Biology of the Heart is to provide recommendations on how to proceed in research with these novel TE strategies combined with cell-based therapies to boost cardiac repair in the clinical settings of IHD and HF.

Development of a functional airway-on-a-chip by 3D cell printing.

We used 3D cell printing to emulate an airway coupled with a naturally-derived blood vessel network in vitro. Decellularized extracellular matrix bioink derived from porcine tracheal mucosa (tmdECM) was used to encapsulate and print endothelial cells and fibroblasts within a designated polycarprolactone (PCL) frame. Providing a niche that emulates conditions in vivo, tmdECM gradually drives endothelial re-orientation, which leads to the formation of a lumen and blood vessel network. A fully-differentiated in vitro airway model was assembled with the printed vascular platform, and collectively reproduced a functional interface between the airway epithelium and the vascular network. The model presented respiratory symptoms including asthmatic airway inflammation and allergen-induced asthma exacerbation in physiological context. Because of the adaptable and automated nature of direct 3D cell printing, we expect that this will have relevance in vivo and high reproducibility for production of high-content platforms for preclinical trials in biomedical research.

Bioengineering the innate vasculature of complex organs: what have we learned so far.

PURPOSE OF REVIEW: Engineering vasculature that meets an organ's specific physiology and function is a fundamental step in organ bioengineering. In this article, we review approaches for engineering functional vasculature for organ bioengineering, with an emphasis on the engineering of organ-specific endothelium and vasculature. RECENT FINDINGS: Recent advances in hydrogel-based engineering of vascularized organ bud enable vascular regeneration in self-assembled cellular niche containing parenchymal and stromal cells. The emerging technology of whole-organ decellularization provides scaffold materials that serve as extracellular niche guiding vascular regeneration to recapitulate native organ's vascular anatomy. Increasing morphological and molecular evidences suggest endothelial heterogeneity across different organs and across different vascular compartments within an organ. Deriving organ-specific endothelium from pluripotent stem cells has been shown to be possible by combining endothelial induction with parenchymal differentiation. SUMMARY: Engineering organ-specific vasculature requires the combination of organ-specific endothelium with its unique cellular and extracellular niches. Future investigations are required to further delineate the mechanisms for induction and maintenance of organ-specific vascular phenotypes, and how to incorporate these mechanisms to engineering organ-specific vasculature.

Regulation of Regenerative Medicine Products.

Cellular therapies have moved to the forefront based upon promising results from clinical trials using both chimeric antigen receptor T lymphocytes to treat leukemia and other cell types to restore structure and function to tissues that have been damaged by disease or physical injury. The pace at which these treatments have evolved has posed a regulatory challenge to agencies, such as the Food and Drug Administration (FDA). This chapter describes how a specific regulatory strategy was developed and how it has evolved in response to the demand for these new therapies.