Biological studies in animal models using [99mTc](CO)3 recombinant annexin V as diagnostic agent of apoptotic processes

[99m Tc]Tc-HYNIC-EcgDf21: A defensin short analogue with potential application in infection foci imaging

Opportunistic infections are a problem of great relevance in public health and the precise detection and localization of infection in the early stages of the disease is of great importance for patient management as well as cost containment. Our proposal seeks to contribute to developing a new agent that meets the needs of diagnosis and follow-up of fungal and bacterial infections, focused on the design of a radiotracer with the potential for recognition of hidden infection foci. Defensins are plant antimicrobial peptides that not only show activity against plant pathogens but also against human ones. A short analogue of EcgDf1 defensin, EcgDf21d (NH₂ -ERFTGGHCRGFRRRCFCTKHC-COOH), was labelled through the formation of a ^99m Tc-HYNIC complex which was assessed for physicochemical and biological behaviour both in vitro and in vivo. The [^99m Tc]Tc-HYNIC-EcgDf21 labelling procedure rendered a single product with remarkably high RCP and stability in the labelling milieu. The Log p value indicated that [^99m Tc]Tc-HYNIC-EcgDf21 has a hydrophilic behaviour, confirmed by the biodistribution profiles. The optimal uptake value was obtained for Candida albicans infection model reaching a lesion/muscle ratio of 3, this correlates with in vitro binding studies, and the lesion can be definitely observed in the scintigraphic images.

Measurement of formation cross-section of 99Mo from the 98Mo(n,γ) and 100Mo(n,2n) reactions

The formation cross-section of medical isotope ⁹⁹Mo from the ⁹⁸Mo(n,γ) reaction at the neutron energy of 0.025eV and from the ¹⁰⁰Mo(n,2n) reaction at the neutron energies of 11.9 and 15.75MeV have been determined by using activation and off-line γ-ray spectrometric technique. The thermal neutron energy of 0.025eV was used from the reactor critical facility at BARC, Mumbai, whereas the average neutron energies of 11.9 and 15.75MeV were generated using ⁷Li(p,n) reaction in the Pelletron facility at TIFR, Mumbai. The experimentally determined cross-sections were compared with the evaluated nuclear data libraries of ENDF/B-VII.1, CENDL-3.1, JENDL-4.0 and JEFF-3.2 and are found to be in close agreement. The ¹⁰⁰Mo(n,2n)⁹⁹Mo reaction cross-sections were also calculated theoretically by using TALYS-1.8 and EMPIRE-3.2 computer codes and compared with the experimental data.

Optimising the radiolabelling properties of technetium tricarbonyl and His-tagged proteins

BACKGROUND

To date, the majority of protein-based radiopharmaceuticals have been radiolabelled using non-site-specific conjugation methods, with little or no control to ensure retained protein function post-labelling. The incorporation of a hexahistidine sequence (His-tag) in a recombinant protein can be used to site-specifically radiolabel with 99mTc-tricarbonyl ([99mTc(CO)3]+). This chemistry has been made accessible via a technetium tricarbonyl kit; however, reports of radiolabelling efficiencies and specific activities have varied greatly from one protein to another. Here, we aim to optimise the technetium tricarbonyl radiolabelling method to produce consistently >95% radiolabelling efficiencies with high specific activities suitable for in vivo imaging.

METHODS

Four different recombinant His-tagged proteins (recombinant complement receptor 2 (rCR2) and three single chain antibodies, α-CD33 scFv, α-VCAM-1 scFv and α-PSMA scFv), were used to study the effect of kit volume, ionic strength, pH and temperature on radiolabelling of four proteins.

RESULTS

We used 260 and 350 μL [99mTc(CO)3]+ kits enabling us to radiolabel at higher [99mTc(CO)3]+ and protein concentrations in a smaller volume and thus increase the rate at which maximum labelling efficiency and specific activity were reached. We also demonstrated that increasing the ionic strength of the reaction medium by increasing [Na+] from 0.25 to 0.63 M significantly increases the rate at which all four proteins reach a >95% labelling efficiency by at least fourfold, as compared to the conventional IsoLink® kit (Covidien, Petten, The Netherlands) and 0.25 M [Na+].

CONCLUSION

We have found optimised kit and protein radiolabelling conditions suitable for the reproducible, fast, efficient radiolabelling of proteins without the need for post-labelling purification.