Early results in the irrational design of new bifunctional chelators

PSMA-GCK01: A Generator-Based 99mTc/188Re Theranostic Ligand for the Prostate-Specific Membrane Antigen

Prostate-specific membrane antigen (PSMA) theranostics have been introduced with 68Ga and 177Lu, the most used radionuclides. However, 188Re is a well-known generator-based therapeutic nuclide that completes a theranostic tandem with 99mTc and may offer an interesting alternative to the currently used radionuclides. In the present work, we aimed at the development of a PSMA-targeted 99mTc/188Re theranostic tandem. Methods: The ligand HYNIC-iPSMA was chosen as the lead structure. Its HYNIC chelator has limitations for 188Re labeling and was replaced by mercaptoacetyltriserine to obtain PSMA-GCK01, a precursor for stable 99mTc and 188Re labeling. 99mTc-PSMA-GCK01 was used for in vitro evaluation of the ligand and comparison with 99mTc-EDDA/HYNIC-iPSMA. Planar imaging using 99mTc-PSMA-GCK01 and organ biodistribution with 188Re-PSMA-GCK01 were performed using LNCaP tumor-bearing mice. Finally, the theranostic tandem was applied for imaging and therapy in 3 prostate cancer patients in compassionate care. Results: Efficient radiolabeling of PSMA-GCK01 with both radionuclides was demonstrated. Cell-based assays with 99mTc-PSMA-GCK01 versus 99mTc-EDDA/HYNIC-iPSMA revealed comparable uptake characteristics. Planar imaging and organ distribution revealed good tumor uptake of both 99mTc-PSMA-GCK01 and 188Re-PSMA-GCK01 at 1 and 3 h after injection, with low uptake in nontarget organs. In patients, similar distribution patterns were observed for 99mTc-PSMA-GCK01 and 188Re-PSMA-GCK01 and in comparison with 177Lu-PSMA-617. Conclusion: The ligand PSMA-GCK01 labels stably with 99mTc and 188Re, both generator-based radionuclides, and thus provides access to on-demand labeling at reasonable costs. Preclinical evaluation of the compounds revealed favorable characteristics of the PSMA-targeted theranostic tandem. This result was confirmed by successful translation into first-in-humans application.

In vitro cellular uptake and dimerization of signal transducer and activator of transcription-3 (STAT3) identify the photosensitizing and imaging-potential of isomeric photosensitizers derived from chlorophyll-a and bacteriochlorophyll-a

Among the photosensitizers investigated, both ring-D and ring-B reduced chlorins containing the m-iodobenzyloxyethyl group at position-3 and a carboxylic acid functionality at position-17(2) showed the highest uptake by tumor cells and light-dependent photoreaction that correlated with maximal tumor-imaging [positron emission tomography (PET) and fluorescence] and long-term photodynamic therapy (PDT) efficacy in BALB/c mice bearing Colon26 tumors. However, among the ring-D reduced compounds, the isomer containing the 1'-m-iobenzyloxyethyl group at position-3 was more effective than the corresponding 8-(1'-m-iodobenzyloxyethyl) derivative. All photosensitizers showed maximum uptake by tumor tissue 24 h after injection, and the tumors exposed with light at low fluence and fluence rates (128 J/cm(2), 14 mW/cm(2)) produced significantly enhanced tumor eradication than those exposed at higher fluence and fluence rate (135 J/cm(2), 75 mW/cm(2)). Interestingly, dose-dependent cellular uptake of the compounds and light-dependent STAT3 dimerization have emerged as sensitive rapid indicators for PDT efficacy in vitro and in vivo and could be used as in vitro/in vivo biomarkers for evaluating and optimizing the in vivo treatment parameters of the existing and new PDT candidates.

99mTc-chelator engineering to improve tumour targeting properties of a HER2-specific Affibody molecule

PURPOSE

Monitoring HER2 expression is crucial for selection of breast cancer patients amenable to HER2-targeting therapy. The Affibody molecule Z(HER2:342) binds to HER2 with picomolar affinity and enables specific imaging of HER2 expression. Previously, Z(HER2:342) with the additional N-terminal mercaptoacetyl-glycyl-glycyl-glycyl (maGGG) sequence was labelled with (99m)Tc and demonstrated specific targeting of HER2-expressing xenografts. However, hepatobiliary excretion caused high radioactivity accumulation in the abdomen. We investigated whether the biodistribution of Z(HER2:342) can be improved by substituting glycyl residues in the chelating sequence with more hydrophilic seryl residues.

METHODS

The Affibody molecule Z(HER2:342), carrying the chelators mercaptoacetyl-glycyl-seryl-glycyl (maGSG), mercaptoacetyl-glycyl-D: -seryl-glycyl [maG(D-S)G] and mercaptoacetyl-seryl-seryl-seryl (maSSS), were prepared by peptide synthesis and labelled with (99m)Tc. The differences in the excretion pathways were evaluated in normal mice. The tumour targeting capacity of (99m)Tc-maSSS-Z(HER2:342) was studied in nude mice bearing SKOV-3 xenografts and compared with the capacity of radioiodinated Z(HER2:342).

RESULTS

A shift towards renal excretion was obtained when glycine was substituted with serine in the chelating sequence. The radioactivity in the gastrointestinal tract was reduced threefold for the maSSS conjugate in comparison with the maGGG conjugate 4 h post injection (p.i.). The tumour uptake of (99m)Tc-maSSS-Z(HER2:342) was 11.5 +/- 0.5% IA/g 4 h p.i., and the tumour-to-blood ratio was 76. The pharmacokinetics and uptake characteristics of technetium-labelled Z(HER2:342) were better than those of radioiodinated Z(HER2:342).

CONCLUSION

The introduction of serine residues in the chelator results in better tumour imaging properties of the Affibody molecule Z(HER2:342) compared with glycyl-containing chelators and is favourable for imaging of tumours and metastases in the abdominal area.

Technetium-99m labeled peptides--an investigation of multiple HPLC peaks

This laboratory, and others, have reported multiple radioactive peaks in the size exclusion high performance liquid chromatographic (HPLC) analysis of 99mTc-labeled peptides. In the case of one 99mTc-MAG3-labeled peptide studied in this laboratory, human neutrophil elastase inhibitor, all five radioactive peaks were shown to be due to active peptide rather than radiocontaminants. By a variety of experiments, the nature of these peaks have now been examined. A high molecular weight UV peak could be generated by heating the MAG3 coupled, but not the native, peptide. Furthermore, this UV peak did not appear upon heating the peptide if the sulfur within the MAG3 chelator was replaced with oxygen. This peak may therefore be due to polymers resulting from intermolecular disulfide bond formation between sulfurs in the MAG3 chelate and the peptide. Several peaks with apparent lower molecular weights were absent on analysis with a different size exclusion column with superior resolution in their molecular weight range. More importantly, they were also absent on analysis by SDS polyacrylamide gel electrophoresis. These "low" molecular weight radioactive peaks may therefore be due to interactions between the 99mTc-MAG3 chelate and the peptide which produce multiple molecular configurations of identical molecular weight but differing in shape, charge, isomerism or lipophilicity such that they are resolved under the conditions of certain analyses. In support of this possibility, lengthening the linker between MAG3 and the peptide reduced the number of radioactive peaks, while encouraging the interaction by replacing MAG3 with the shorter MAG2 seemed to increase the number of radioactive peaks. Finally, that the three "low" molecular weight radioactive peaks reappeared when a single peak fraction was reanalyzed suggests that the species responsible are in rapid equilibrium. One conclusion from this investigation is that the appearance of a single peak by any HPLC analysis offers no assurance that multiple peaks would not appear on alternative HPLC analyses. Evidence that each species is due to radiolabeled active peptide and not to radiocontaminants is therefore potentially more important than evidence of a single peak.

NHS-MAS3: a bifunctional chelator alternative to NHS-MAG3

This laboratory uses an N-hydroxysuccinimide derivative of S-acetylmercaptoacetyltriglycine (NHS-MAG3) to conjugate amines for subsequent labeling with 99mTc. However, the synthesis from triglycerine is general and not restricted to this tripeptide. We had earlier selected a small number of alternative tripeptides and synthesized the corresponding NHS derivatives. Each was then evaluated in a search for bifunctional chelators with properties superior to NHS-MAG3, such as lower serum protein binding or improved stability to cysteine challenge. Based on these preliminary results, NHS-S-acetylmercaptoacetyltriserine (NHS-MAS3) was selected for further investigation. We have now conjugated this bifunctional chelator to an biocytin and to an amine-derivatized peptide nucleic acid (PNA). Both carriers were also conjugated with NHS-MAG3 under identical conditions and all were labeled with 99mTc at neutral pH and at boiling temperature while the conjugated PNAs were radiolabelled at neutral pH and at room temperature. Regardless of the chelator, reverse phase HPLC radiochromatograms of the labeled biotins and PNAs after purification showed a single peak. However, by size exclusion HPLC, the radiochromatograms always showed several peaks even after purification, but the MAS3 radiochromatograms were less complicated. For biotin and PNA both, radiolabeling via MAS3 showed improved 99mTc stability in 37 degrees C serum and in cysteine solution. The four preparations were administered to mice implanted in one thigh with avidin beads (biotins) or complementary PNA beads (PNAs). At 5 h post-administration, no significant differences were observed in the targeting of PNA beads between the two chelators, however the target thigh/normal thigh ratio was significantly higher for MAS3-biotin compared to MAG3-biotin. We conclude that labeling biocytin and amine-derivatized PNA with NHS-MAS3 compared to NHS-MAG3 provides simpler radiochromatographic profiles, improved stability of the label in serum and cysteine solution and can improve in vivo targeting.

The influence of temperature and alkaline pH on the labeling of free and conjugated MAG3 with technetium-99m

Benzoyl-protected mercaptoacetyltriglycine (S-benzoyl MAG3) is radiolabeled by tartrate transchelation at elevated temperatures or basic pH. The object of this investigation was to establish whether the same 99mTc labeled species are formed when S-acetyl (S-acetyl MAG3)-conjugated compounds are radiolabeled by tartrate transchelation at ambient temperature and neutral pH in contrast to labeling at 95 degrees C and pH 11. S-acetyl MAG3 was conjugated to biocytin and to the amine-derivitized oligomers DNA and PNA. Along with free S-acetyl MAG3, these were radiolabeled under the different conditions. Although labeling efficiencies were always lower when labeled at ambient temperature and neutral pH relative to labeling at 95 degrees C or pH 11 (free S-acetyl MAG3 could not be labeled at all), size exclusion and reverse phase HPLC showed no difference with labeling conditions in the radiochemical profiles for labeled DNA and biocytin. In the case of DNA, a cysteine challenge also failed to demonstrate a difference. However, in the case of PNA, some important differences were observed in the size exclusion HPLC radiochromatograms. In addition, PNA labeled at ambient temperature and neutral pH was less stable to transchelation to cysteine. In conclusion, S-acetyl MAG3 conjugated compounds may be radiolabeled at ambient temperature and neutral pH. In most cases, the radiochemical species produced appear to be identical to those formed when labeling is accomplished at 95 degrees C or pH 11.