To evaluate biodistribution and tumor binding in patients TRACER performed the labeling of a scFv antibody fragment with zirconium-89 (89Zr). TRACER managed the complete process, from labeling strategy to clinical production to Qualified Person (QP) release of the radiolabeled material under Good Manufacturing Practice (GMP) and International Conference on Harmonization for Good Clinical Practice (ICH-GCP).
89Zr-radioimmunoconjugates
Antibody labeling for PET imaging with 89Zr is a common practice, it is also known as ⁸⁹Zr-immuno-PET. For this study TRACER selected the DFO-N-suc chelator, which is one of several frequently chosen chelator strategies to use antibodies as a PET tracer for imaging. In the labeling process, we considered factors such as the fact that antibody fragments are cleared faster than whole antibodies. To extend half-life, the scFv fragments have been fused to non-human albumin-binding peptides.
89Zr-DFO-N-Suc-scFv
The bifunctional chelator DFO-N-Suc enables radiolabeling of proteins. One end of the molecule chelates a metal ion, in this case zirconium-89 (⁸⁹Zr), while the other end forms a covalent bond with proteins such as an scFv antibody fragment. The N-succinyl group allows conjugation to lysine residues on the protein via amide bond formation.
DFO: Desferrioxamine (a siderophore and metal chelator)
N-Suc: N-succinyl (a functional group enabling conjugation to proteins)
ScFv: Single chain Fragment variable (antibody fragment)
From GLP drug substance to GMP labeled drug product
The scFv fragments were produced under non-GMP conditions and used for the conjugation and radiolabeling under GMP conditions in a radiopharmacy. As a microdose was used in the Phase 0 study, the requirements for the scFv were less strict. Regulatory guidelines on GMP labeling of a Good Laboratory Practices (GLP) drug substance are not available, but in practice, this method has been accepted by the authorities. Together with the qualified person (QP), a risk assessment was conducted and translated into method qualification and validation steps for the radiolabeled drug product. After manufacturing and testing, the QP release follows, resulting in a GMP drug product suitable for clinical use.
Download requirements for manufacturing of biological intermediates for a microdosing in-human trial
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Study approval and logistics
Studies with tracers designed and manufactured according to the 89Zr-DFO method have been evaluated in patients before, which is favorable for fast regulatory approval. The half-life of 78.4 h aligns well with that of antibodies, while allowing flexibility in tracer shipment and handling. TRACER also conducted the shipment of the tracer to the clinical site and provided a pharmacy manual with detailed storage conditions to ensure safety and stability.