PNA and affinity protein tools for selective tumor targeting of radiopharmaceuticals
Time: Fri 2022-10-07 10.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Subject area: Biotechnology
Doctoral student: Hanna Tano , Proteinvetenskap
Opponent: Prof. Dr. Dario Neri, Department of Chemistry and Applied Biosciences, ETH Zürich
Supervisor: Professor Amelie Eriksson Karlström, Albanova VinnExcellence Center for Protein Technology, ProNova, Proteinvetenskap, Proteinteknologi; Professor Per-Åke Nygren, Skolan för kemi, bioteknologi och hälsa (CBH), Albanova VinnExcellence Center for Protein Technology, ProNova
Targeted radiotherapy of cancer intends to selectively deliver cytotoxic radionuclides to tumor cells. Affinity proteins of various kinds are explored for this task, and depending on the affinity protein used, different challenges arise. Full-length antibodies are typically associated with long serum half-life, leading to high systemic toxicity, while smaller affinity ligands such as engineered scaffold proteins, antibody fragments or peptides, usually demonstrate high radioactive uptake in kidneys. The smallest affinity ligands furthermore suffer from low therapeutic efficacy due to their fast wash-out, thus demanding frequent administrations of the radio-conjugate to reach a therapeutic effect.
These issues were addressed in this thesis, where small affinity ligands (an Affibody molecule, a single domain antibody fragment and a peptide) have been explored as targeting agents for the cancer targets HER2, CD38 and GRPR, respectively. The Affibody molecule and the single domain antibody fragment were used in a pretargeting setting where high selective hybridization are used as recognition tags between peptide nucleic acid (PNA) strands on the tumor targeting primary agent and the radiolabelled secondary agent. In papers I and II, different sets of PNA hybridization probes were evaluated, in vitro and in vivo. In paper I, we demonstrate that the shortest tested secondary PNA probe (the 9-mer HP16) had the most favourable biodistribution profile with high tumor uptake along with the lowest kidney uptake. In paper II, we produced a set of shorter primary PNA probes, aiming for simplified production, and new sets of even shorter secondary PNA probes. A secondary 8-mer was identified as suitable for testing in cell assays and in vivo together with HER2-binding Affibody-PNA conjugates with varying length of the primary PNA probe, in order to determine if the smaller hydrodynamic range would further improve the biodistribution properties. In paper III, the Affibody-mediated PNA-based pretargeting strategy was evaluated as a monotherapy and as a co-treatment strategy with trastuzumab, to treat mice bearing HER2-positive tumors. Mice treated with the co-treatment strategy had significantly longer survival compared to other groups. In paper IV, the feasibility of using the PNA pretargeting strategy in combination with another affinity protein (a single domain antibody fragment) was evaluated in a CD38-expressing cell line. In paper V, the GRPR-binding peptide RM26 was conjugated to an albumin-binding domain, with the aim to achieve a high tumor uptake over time. The RM26-ABD conjugate did demonstrate good tumor uptake over time. However, the conjugate also demonstrated high kidney uptake, limiting its use as a therapeutic construct.
In conclusion, the work presented in this thesis shows strategies for selective tumor targeting of radiopharmaceuticals using affinity proteins and PNA-mediated pretargeting.