Peptide-mediated targeting of cytokines to tumor vasculature: the NGR-hTNF example

Abstract

A growing body of evidence suggests that the efficacy of cytokines in cancer therapy can be increased by targeting strategies based on conjugation with ligands that recognize receptors expressed by tumor cells or elements of the tumor microenvironment, including the tumor vasculature. The targeting approach is generally conceived to permit administration of low, yet pharmacologically active, doses of drugs, thereby avoiding toxic reactions. However, it is becoming clear that, in the case of cytokines, this strategy has another inherent advantage, i.e. the possibility of administering extremely low doses that do not activate systemic counter-regulatory mechanisms, which may limit their potential therapeutic effects. This review is focused on the use of tumor vasculature-homing peptides as vehicles for targeted delivery of cytokines to tumor blood vessel. In particular, we provide an overview of peptide-cytokine conjugates made with peptides containing the NGR, RGD, isoDGR or RGR sequences and describe, in more details, the biological and pharmacological properties of NGR-hTNF, a peptide-tumor necrosis factor-α conjugate that is currently being tested in phase II and III clinical studies. The results of preclinical and clinical studies performed with these products suggest that peptide-mediated vascular-targeting is indeed a viable strategy for delivering bioactive amounts of cytokines to tumor endothelial cells without causing the activation of counter-regulatory mechanisms and toxic reactions.

Fig. 1

Effect of NGR-TNF and TNF on tumor growth and soluble TNF receptor shedding, an important counter-regulatory mechanism. a Effect on the growth of RMA-T lymphomas implanted subcutaneously in mice (tumor volumes, 4 days after treatment, showing different dose-response curves for NGR-TNF and TNF) [42]. b Effect of low (0.1 ng/mouse), moderate (10 ng), and high (1,000 ng) doses of NGR-TNF or TNF on the shedding of soluble TNF-R2 (serum levels, 1 h after treatment). Strong activation of this counter-regulatory mechanism occurs only with moderate and high doses of both cytokines. c Schematic representation of the hypothetical interactions of low, moderate, and high doses of NGR-TNF or TNF with soluble and membrane TNF receptors in normal (CD13-negative) and tumor-associated vessels (CD13-positive). This model can explain the different anti-tumor dose-response curves observed (adapted from [42]). According to this model, moderate doses of NGR-TNF or TNF (e.g. 10 ng/mouse), can efficiently interact with membrane TNF receptors throughout the body and trigger the systemic release of sTNF-Rs, which act as potent TNF inhibitors. High doses of both cytokines (e.g. 1,000–10,000 ng) can saturate the circulating sTNF-Rs, thereby leaving a significant amount of bioactive TNF free to interact with membrane receptors in neoplastic and normal tissues and to induce, therefore, anti-tumor as well as toxic effects. In contrast, low-dose NGR-TNF or TNF (e.g. 0.1 ng) cannot induce massive soluble receptor shedding, likely because of insufficient interaction with endothelial cells in normal vessels. However, in this case, NGR-TNF can still induce anti-tumor effects owing to high-avidity interaction with membrane TNF receptors and CD13 on tumor vessels. TNF tumour necrosis factor