Perkins, Alan and Missailidis, Sotiris
Radiolabelled aptamers for imaging and therapy.
World Journal of Nuclear Medicine, 4(2),
The growth in biotechnology has led to new techniques for the design, selection and production of ligands capable of molecular recognition. One promising approach is the production of specific receptor binding molecules based on specific nucleic acid sequences that are capable of recognising a wide array of target molecules. These oligonuclide ligands are known as aptamers (1.2). The technology that allows production of aptamer molecules is known as systematic evolution of ligands by exponential enrichment (SELEX). We have used combinatorial chemistry techniques coupled with polymerase chain reaction (PCR) to rapidly select aptamers from degenerate libraries that bind with high affinity and specificity to the protein core of the MUC1 antigen, a tumour marker previously extensively used in tumour imaging and therapy. MUC1 is widely expressed by normal glandular epithelial cells. However this expression is dramatically increased when the cells become malignant. This has been well documented for breast and ovarian cancer, as well as some lung, pancreatic and prostate cancers (3). Recently it has also been shown that MUC1 is a valuable marker for bladder and has been used for the imaging and targeted therapy of bladder cancer.
The aptamer selection process was performed on affinity chromatography matrices. After ten rounds of selection and amplification, aptamers were cloned and sequenced. Post SELEX amino modifications have been used to confer nuclease resistance and coupling potential. The aptamers bound to MUC1 antigen with a Kd of 5nm and high specificity, demonstrated by fluorescent microscopy on MUC1-expresing tumour cells. Using peptide coupling reactions, we have successfully attached chelators for Tc-99m radiolabelling. Two of the constructs tested were based on mono-aptamer chelator complexes, one with commercially available MAG3 and one with a novel designed cyclen-based chelator. The other two constructs were based on the use of multi-aptamer complexes, where four aptamers were attached to the four arms of either DOTA or carboxy-porphyrin.
The four complexes were labelled with Tc-99m and tested for their efficacy as tumour imaging agents. All four complexes demonstrated specificity for the tumour, due to their MUC1 specificity, at various levels. Biodistribution studies were carried out in mice with MCF7 xenografts. The monomeric aptamer complexes had rapid renal clearance from the system, due to their small size (MW of 8,000 Da). More than 90% of the aptamer based radiopharmaceutical was cleared from the system within the first 15 minutes. To increase retention time, additional constructs based on the design of a tetra-aptamer complex were prepared. A core chelator, such as DOTA and carboxy-porphyrin has been used as a skeleton for the building of multiaptamer constructs aiming to increase the molecular weight of the complex and potentially its stability of binding due to interactions with more than one MUC1 molecules at the surface of the tumour cell. The increase of the MW to 32,000 Da allowed increased retention times in the system, without compromising the exceptional tumour penetration exhibited by all the aptamer based constructs under study.
The development of aptamers as small building blocks for targeting agents offers several advantages. These molecules penetrate tumour more readily than whole antibodies, reach peak levels in the tumour more rapidly and clear from the body faster, thereby reducing toxicity to healthy tissues. Our strategy is to manipulate the molecular weight of the construct utilising previously devised methodologies to achieve various polymeric aptamer complexes in order to achieve the optimum balance between the low immunogenicity and excellent tumour penetration. In this way we aim to achieve a balance against the rapid renal clearance that leads to premature elimination of the complex from the system and adequate tumour uptake for diagnostic imaging and targeted therapy. We intend to undertake further work using Re-188 to produce a therapeutic aptamer conjugate.
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