99mTc-D(RGD): molecular imaging probe for diagnosis of αvβ3-positive tumors

Topoisomeric Membrane-Active Peptides: A Review of the Last Two Decades

In recent decades, bioactive peptides have been gaining recognition in various biomedical areas, such as intracellular drug delivery (cell-penetrating peptides, CPPs) or anti-infective action (antimicrobial peptides, AMPs), closely associated to their distinct mode of interaction with biological membranes. Exploiting the interaction of membrane-active peptides with diverse targets (healthy, tumoral, bacterial or parasitic cell membranes) is opening encouraging prospects for peptides in therapeutics. However, ordinary peptides formed by L-amino acids are easily decomposed by proteases in biological fluids. One way to sidestep this limitation is to use topoisomers, namely versions of the peptide made up of D-amino acids in either canonic (enantio) or inverted (retroenantio) sequence. Rearranging peptide sequences in this fashion provides a certain degree of native structure mimicry that, in appropriate contexts, may deliver desirable biological activity while avoiding protease degradation. In this review, we will focus on recent accounts of membrane-active topoisomeric peptides with therapeutic applications as CPP drug delivery vectors, or as antimicrobial and anticancer candidates. We will also discuss the most common modes of interaction of these peptides with their membrane targets.

Synthesis of Novel Carborane-Containing Derivatives of RGD Peptide

Short peptides containing the Arg-Gly-Asp (RGD) fragment can selectively bind to integrins on the surface of tumor cells and are attractive transport molecules for the targeted delivery of therapeutic and diagnostic agents to tumors (for example, glioblastoma). We have demonstrated the possibility of obtaining the N- and C-protected RGD peptide containing 3-amino-closo-carborane and a glutaric acid residue as a linker fragment. The resulting carboranyl derivatives of the protected RGD peptide are of interest as starting compounds in the synthesis of unprotected or selectively protected peptides, as well as building blocks for preparation of boron-containing derivatives of the RGD peptide of a more complex structure.

Evaluation of 99m Tc-3PRGD2 SPECT imaging on angiogenesis in animal models of lung cancer

BACKGROUND

The main purpose of the study was to evaluate the activity and selectivity of ^99m Tc-3PRGD₂ SPECT/CT and ¹⁸ F-FDG PET-CT in order to detect the neovascularization of A549 cell subcutaneously transplanted tumors, and clarify the relationship among tumor vasculature, hypoxia and cell proliferation in the tumor microenvironment.

METHODS

We established a subcutaneous tumor model, and used ^99m Tc-3PRGD₂ SPECT/CT and ¹⁸ F-FDG PET-CT when the average tumor size reached 0.3-0.5 cm³ . The mice were anesthetized and sacrificed and the tumors were completely removed for frozen section analysis. We subsequently evaluated the status of neovascularization, hypoxia, as well as cell proliferation via immunofluorescence staining (IF) by detecting CD31, pimonidazole and EdU, respectively.

RESULTS

There was a significant positive correlation (r = 0.88, p < 0.05) between the microvascular density (41.20 ± 18.60) and tumor to nontumor ratio (T/M), which was based on the value of ^99m Tc-3PRGD₂ (4.20 ± 1.33); meanwhile, no significance (r = -0.16, p > 0.05) was found between the T/M and hypoxic area (116.71 ± 9.36). Neovascular proliferation was particularly vigorous in the parenchymal region of the tumor, while the cells around the cavity were generally hypoxic. ^99m TC-3PRGD₂ SPECT/CT was more specific than ¹⁸ F-FDG PET-CT in detecting malignant tumors.

CONCLUSION

Both ^99m TC-3PRGD₂ and ¹⁸ F-FDG PET-CT can be used for the detection of malignant tumors, but the specificity and accuracy of ^99m TC-3PRGD₂ are better. The subcutaneous tumors showed a heterogeneous microenvironment as a result of neovascularization, a high proliferation rate of cancer cells as well as subsequent hypoxia, while most of the hypoxic areas appeared around the cavities of the vessels.

[99mTc]-labeling and evaluation of a new linear peptide for imaging of glioblastoma as a αvβ3-positive tumor

PURPOSE

In this study, we designed a new linear 6-Hydrazinonicotinamide (HYNIC)-conjugated peptide (HYNIC-KRWrNM) (M-6) and labeled with technetium-99m for gamma imaging of glioblastoma as a α_vβ₃-positive tumor. We evaluated tumor targeting ability of this radio-peptide and compared with previous ^99mTc-labeled HYNIC-conjugated RGD analogue peptides.

PROCEDURES

One new linear peptide (HYNIC-KRWrNM) (M-6) was designed and labeled with technetium-99m in the presence of 2-[[1,3-dihydroxy-2-(hydroxymethyl) propan-2-yl] amino] acetic acid (Tricine)/Ethylenediamine-N,N'-diacetic acid (EDDA) as co-ligand system. Then, this ^99mTc-labeled peptide ([^99mTc]Tc-M-7) was evaluated for in vitro stability in saline and serum, specific binding assay, internalization, and binding affinity (K_d). In addition, we performed biodistribution study and planar imaging on nude mice bearing U87-MG xenograft as a α_vβ₃-positive tumor.

RESULTS

The radiochemical yield of [^99mTc]Tc-M-7 was obtained ˃95%. This ^99mTc-labeled peptide remained stable and intact in saline solution after 24 h incubation. In addition, metabolic stability of this ^99mTc-labeled peptide was obtained ˃60% after 4 h incubation in serum. The K_d value for [^99mTc]Tc-M-7 was obtained 5.2 ± 1.0 nM. Based on biodistribution results in nude mice bearing U87-MG xenograft, tumor/muscle activity ratio was 6.22 and decreased to 1.89 in blocking group at the same time point (4 h p.i.). The blocking experiment results also indicated that tumor uptake and kidney uptake were α_vβ₃-mediated. In comparison with previous HYNIC-conjugated RGD analogue peptides, kidneys had the highest uptake of this ^99mTc-labeled peptide (52.29 ± 11.48 at 1.5 h p.i. and 27.04 ± 0.66%ID/g at 4 h p.i.). Finally, similar to previous ^99mTc-labeled HYNIC-conjugated RGD analogue peptides, [^99mTc]Tc-M-7 showed acceptable tumor uptake after 4 h post-injection (based on ROI technique, target-to-background activity ratio = 3.80).

CONCLUSIONS

This small linear ^99mTc-labeled peptide, with high affinity to α_vβ₃ integrin, desirable water solubility, and cost efficient, demonstrates a potent tumor targeting ability as well as previous HYNIC-conjugated RGD analogue peptides. Hence, [^99mTc]Tc-M-7 can be of service to as a new candidate for early detection of α_vβ₃-positive tumors.

Study of the 99m Tc-labeling conditions of 6-hydrazinonicotinamide-conjugated peptides from a new perspective: Introduction to the term radio-stoichiometry

Specific tumor uptake of peptide radiopharmaceuticals depends on tumor binding affinity and their radiochemical purity. Several important parameters that influence the 99m Tc-labeling and consequently the radiochemical purity of 6-hydrazinonicotinamide (HYNIC)-conjugated peptide are radionuclide activity, the amount of peptide, the amount of coligands, and the amount of reducing agents (stannous ion). In this review article, we have attempted studying these parameters in the HYNIC-conjugated peptides (somatostatin, cholecystokinin/gastrin, bombesin, and RGD analogs) from a new perspective to obtain most used and optimized radio-stoichiometric relationships. One of the most important results in this review is that for 99m Tc-labeling of HYNIC-conjugated peptides, it is better to consider the most calculated mole ratio between technetium-99m and the peptide (mole ratio of technetium-99m to the peptide 1:200-400). The statistical results also show that among these 99m Tc-labeled peptides, the most used and favorable coligand is tricine/EDDA with two to one (2:1) mole ratio. These optimized radio-stoichiometric relationships, favorable coligand mole ratio, and applicable radiolabeling points can greatly improve the labeling process of the HYNIC-conjugated peptides, by reducing trial and error, increasing specific activity, and saving materials.