Low-molecular-weight radiopharmaceuticals (or radioligands), including small peptides and peptidomimetics, have recently shown great potential for targeting malignancies (e.g., prostate carcinoma, neuroendocrine tumors, etc.) that overexpress distinct surface receptors. Nevertheless, these therapeutic agents encounter a significant biological barrier: accumulation in the kidneys. Due to their small size, they readily pass through the glomerular filtration barrier and undergo active reabsorption in renal cells. This process leads to localized radiation damage, compromised kidney function, and potentially to organ failure. Existing methods to mitigate renal absorption of low-molecular-weight radioligands remain inadequate. Consequently, there is an urgent demand for innovative, widely adaptable strategy to significantly increase the tumor/kidney ratio for therapeutic peptide and/or peptidomimetic radioligands.
According to an excellent review1 by Cheal and colleagues: “Pretargeted radioimmunotherapy (PRIT) is a groundbreaking achievement because its components can be delivered in a time sequence that maximizes radiation to tumor while minimizing radiation to normal tissues”.
First, second-order rate constants (k2) characterize various pretargeting reactions. Our analysis shows that for purely chemical bioorthogonal reactions like rapid inverse-electron demand Diels-Alder (Tz–TCO) and fast SPAAC (azide–alkyne), representing the 2000 to 0.17 M-1s-1 spectrum, virtually no gain in the tumor/kidney ratio is achieved. Staudinger ligation (k2 ~10-3 M-1s-1) and oxime ligation (k2 ~10-4 M-1s-1) are not typically used in vivo, whereas fast copper-based azide–alkyne cycloaddition reaction (CuAAC, k2 ~10 M-1s-1) has issues with copper requirement and its toxicity, which severely limits CuAAC application.
Second, bioengineered streptavidin–biotin system with even higher k2-constant (3x106–4.5x107 M-1s-1), but still low tumor/kidney ratio.
Third, bioengineered tumor-specific anti-CEA × anti-HSG bsMAb-pretargeting and subsequent DOTA-peptide-hapten (IMP-288 and others) injection approach, however, has demonstrated excellent tumor/kidney ratios both in pre-clinical (tumor/kidney ratios 6.6–15) and in clinical studies up to phase 2 (tumor/kidney ratios exceeding 20). Even though this approach is transferable to other cancer types by substituting the anti-CEA module in the anti-CEA × anti-HSG pretargeting bsMAb, the anti-HSG/HSG interaction pair might be proprietary and the resulting bsMAb is a very large molecule by far exceeding 50 kDa (TF2 ~150 kDa). This latter disadvantage — the size of bsMAb — makes this interaction pair not feasible.
References
In the current Solution, novel hybrid pretargeting platform pRadioPeptide, based on both bioengineering and fast chemical reaction, is proposed. pRadioPeptide platform is based on a proven, fast (k2 ~ 1 M-1s-1) chemical biology reaction, which is designed to take place in vivo.
We describe the "Moiety-X"-radioligand-targeted “Motif-Y”-tagged therapeutic peptide (pRadioPeptide) platform. It was proved that the XXXXXXX reaction at the Motif-Y is site-specific (selective), efficient, and generating high yield of the product under physiological conditions.
The XXXXXXX reaction is designed to take place in vivo (in a living organism: human, non-human primate, rat, mouse, etc.). First, the Peptide of Interest (R)–Motif-Y-Conjugate (Compound 10) is injected intravenously (either as single or as multiple injections) to pretarget tumor for 2–24 hours. Second, the [177Lu]-Probe (Compound 9) is injected intravenously (either as single or as multiple injections). Third, the covalent XXXXXXX reaction between the 10 and 9 takes place in vivo to form the radiotherapeutic ligation product, the R–Motif-Y-Conjugate–[177Lu]-Probe (Compound 11, the pRadioPeptide), at the tumor(s) site(s).