In the present study, no dose-limiting toxicities were observed and the combination of low-dose NGR–hTNF with the standard-dose doxorubicin was safe and well tolerated, without an apparent exacerbation of the well-known toxicity profile associated to doxorubicin. In particular, NGR–hTNF did not appear to increase the frequency or severity of doxorubicin-related cardiac toxicity as measured by LVEF. Further analysis of this toxicity, however, would necessitate longer treatment durations in a larger patient population, monitoring of serial B-type natriuretic peptide and troponin levels, and the assessment of QT interval prolongation (Carver et al, 2007
Moreover, only 11% of adverse events were considered to be related to NGR–hTNF, and it should be noted that these events were limited to manageable, short-lasting, and infusion time-related constitutional symptoms.
The combination of molecularly targeted agents with chemotherapy raises issues about the dose selection during early-stage clinical development. Indeed, some of the guideposts in combining chemotherapeutic agents (e.g., definition of MTD and avoidance of overlapping toxicities) might not be suitable when combined with biological agents that have low-toxicity profiles when used at the low-dose range. Moreover, given their mainly cytostatic nature, maximal anti-tumour activity could not necessarily coincide only with MTD, but also with lower drug dose, that is, the optimal biological dose (Kwak et al, 2007
). Accordingly, the rationale of this phase I study was based on a preclinical model showing that even minute amounts of murine NGR–TNF were able to induce synergistic anti-tumour activity in combination with doxorubicin, mainly by damaging the tumour capillary network and increasing the chemotherapeutic drug uptake in the tumour (Curnis et al, 2002
Although not directly measured in this study, the anti-vascular effects of NGR–hTNF have been previously observed by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) in the dose-escalation phase I trial (van Laarhoven et al, 2008
) and in an additional single-agent phase I trial further exploring the low-dose range (Gallo-Stampino et al, 2007
) and, therefore, could be considered supportive of the putative mechanism of synergism shown by NGR–hTNF and doxorubicin. However, the synergism of anti-vascular agents and chemotherapy could be alternatively attributable simply to targeting two distinct cell populations (Horsman and Siemann, 2006
Notwithstanding the fact that optimal biological dose selection is challenging, it is interesting to note that the aforementioned low-dose single-agent phase I trial selected the dose of 0.8μ
for further development, based on a more pronounced anti-vascular effect observed at this dose and the lack of shedding of soluble TNF-receptors registered up to this dose level. These soluble receptors can compete for TNF-α
with the cell-surface receptors, thus blocking its bioavailability and activity, with the amount and speed of receptor shedding being linearly correlated with the serum TNF-α
levels (Aderka et al, 1998
). Similarly, no significant shedding of circulating TNF-α
receptors was observed up to the dose of 0.8μ
in this study. Furthermore, patients enrolled in this dose cohort experienced a low incidence of grade 3–4 toxicity and promising disease control.
Even though anti-tumour activity was not a primary end point of this study, the high disease control rate (73%) achieved in a population heavily pre-treated with chemotherapy, including 9 patients (60%) with an anthracycline-based regimen, seem to be promising by also taking into account the minimal toxicity profile associated to NGR–hTNF and the apparent absence of overlapping toxicity with doxorubicin.
In conclusion, the present phase I trial showed that the combination of low-dose NGR-hTNF and standard-dose doxorubicin is feasible, safe, and well tolerated when administered to patients heavily pre-treated with chemotherapy, including anthracyclines. The observed safety profile and anti-tumour activity warrant further phase II clinical exploration of NGR–hTNF 0.8μ
and doxorubicin 75
in anthracycline-sensitive solid tumours.