A structure-guided fragment-based approach was used to target the lipophilic allosteric binding site of Mycobacterium tuberculosis EthR. This elongated channel has many hydrophobic residues lining the binding site, with few opportunities for hydrogen bonding. We demonstrate that a fragment-based approach involving the inclusion of flexible fragments in the library leads to an efficient exploration of chemical space, that fragment binding can lead to an extension of the cavity, and that fragments are able to identify hydrogen-bonding opportunities in this hydrophobic environment that are not exploited in Nature. In the present paper, we report the identification of a 1 μM affinity ligand obtained by structure-guided fragment linking.
Bioorthogonal chemistry represents a challenging approach in pretargeted radioimmunotherapy (PRIT). We focus here on mAb modifications by grafting an increase amount of trans-cyclooctene (TCO) derivatives (0 to 30 equivalents with respect to mAb) bearing different polyethylene glycol (PEG) linkers between mAb and TCO (i.e. PEG0 (1), PEG4 (2) and PEG12 (3)) and assessing their functionality. We used colorectal xenograft (HT29/Ts29.2) and peritoneal carcinomatosis (A431-CEA-Luc/35A7) as tumor cells/mAbs models and fluorescent tetrazines (TZ). MALDI-TOF MS shows that grafting with 2,3 increases significantly the number of TCO per mAb compared with no PEG. In vitro immunofluorescence showed that Ts29.2 and 35A7 labeling intensity is correlated with the number of TCO when using 1,3 while signals reach a maximum at 10 equivalents when using 2. Under 10 equivalents conditions, the capacity of resulting mAbs-1–3 for antigen recognition is similar when reported per grafted TCO and comparable to mAbs without TCO. In vivo, on both models, pretargeting with mAbs-2,3 followed by TZ injection induced a fluorescent signal two times lower than with mAbs-1. These findings suggest that while PEG linkers allow a better accessibility for TCO grafting, it might decrease the number of reactive TCO. In conclusion, mAb-1 represents the best candidate for PRIT.
Rationale: Pretargeted radioimmunotherapy (PRIT) based upon bioorthogonal click chemistry has been investigated for the first time in the context of peritoneal carcinomatosis using a CEA-targeting 35A7 mAb bearing trans-cyclooctene (TCO) moieties and several 177Lu-labeled tetrazine (Tz) radioligands. Starting from three Tz probes containing PEG linkers of varying lengths between the DOTA and Tz groups (i.e. PEGn = 3, 7, or 11, respectively, for Tz-1, Tz-2, and Tz-3), we selected [177Lu]Lu-Tz-2 as the most appropriate for pretargeted SPECT imaging and demonstrated its efficacy in tumor growth control. Methods: An orthotopic model of peritoneal carcinomatosis (PC) was obtained following the intraperitoneal (i.p.) injection of A431-CEA-Luc cells in nude mice. Tumor growth was assessed using bioluminescence imaging. Anti-CEA 35A7 mAb was grafted with 2-3 TCO per immunoglobulin. Pretargeted SPECT imaging and biodistribution experiments were performed to quantify the activity concentrations of [177Lu]Lu-Tz-1-3 in tumors and non-target organs to determine the optimal Tz probe for the PRIT of PC. Results: The pharmacokinetic profiles of [177Lu]Lu-Tz-1-3 alone were determined using both SPECT imaging and biodistribution experiments. These data revealed that [177Lu]Lu-Tz-1 was cleared via both the renal and hepatic systems, while [177Lu]Lu-Tz-2 and [177Lu]Lu-Tz-3 were predominantly excreted via the renal system. In addition, these results illuminated that the longer the PEG linker, the more rapidly the Tz radioligand was cleared from the peritoneal cavity. The absorbed radiation dose corresponding to pretargeting with 35A7-TCO followed 24 h later by [177Lu]Lu-Tz-1-4 was higher for tumors following the administration of [177Lu]Lu-Tz-2 (i.e. 0.59 Gy/MBq) compared to either [177Lu]Lu-Tz-1 (i.e. 0.25 Gy/MBq) and [177Lu]Lu-Tz-3 (i.e. 0.18 Gy/MBq). In a longitudinal PRIT study, we showed that the i.p. injection of 40 MBq of [177Lu]Lu-Tz-2 24 hours after the systemic administration of 35A7-TCO significantly slowed tumor growth compared to control mice receiving only saline or 40 MBq of [177Lu]Lu-Tz-2 alone. Ex vivo measurement of the peritoneal carcinomatosis index (PCI) confirmed that PRIT significantly reduced tumor growth (PCI = 15.5 ± 2.3 after PRIT vs 30.0 ± 2.3 and 30.8 ± 1.4 for the NaCl and [177Lu]Lu-Tz-2 alone groups, respectively). Conclusion: Our results clearly demonstrate the impact of the length of PEG linkers upon the biodistribution profiles of 177Lu-labeled Tz radioligands. Furthermore, we demonstrated for the first time the possibility of using bioorthogonal chemistry for both the pretargeted SPECT and PRIT of peritoneal carcinomatosis.
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