Melphalan availability in hypoxia‐inducible factor‐1α^+/+ and factor‐1α^–/– tumors is independent of tumor vessel density and correlates with melphalan erythrocyte transport

Abstract

Many tumors are impervious to anticancer agents. Resistance due to lack of cytotoxic penetration into the tumor is often overlooked but can play a significant role in undermining therapy. Insufficient and irregular vascularization of tumors is a possible barrier to drug delivery, but there are others, e.g., impaired vessel permeability and poor interstitial transport. We evaluated the importance of tumor vessel density in the availability of melphalan to tumors. A nude mouse tumor model with different vascularization due to a single-gene deletion of hypoxia-inducible factor 1␣ (HIF-1␣ ؉/؉ and HIF-1␣ -/-embryonic stem cell-derived tumors) was used. The availability of melphalan to HIF-1␣ ؉/؉ (n ‫؍‬ 20) and HIF-1␣ -/-(n ‫؍‬ 23) tumors was not significantly different (p ‫؍‬ 0.12). Furthermore, in the various subgroup analyses accentuating the difference in vessel density, no significant correlation between vessel density and melphalan availability was found. In the second part of the study, melphalan, was demonstrated to be transported in blood in mice with a distribution of 24% in erythrocytes vs. 76% in plasma. A strong correlation (r ‫؍‬ 0.93, p < 0.000001) between melphalan concentrations in plasma and erythrocytes was found, indicating an equilibrium between these 2 compartments. Plasma and erythrocyte concentrations of melphalan are correlated with the tumor availability of melphalan (r ‫؍‬ 0.66 and 0.64, respectively, both p < 0.001). These data suggest that tumor vessel density is not an important predictor of the tumor availability of small cytotoxic drugs such as melphalan and indicate the importance of erythrocytes in the transport of melphalan.

Effect of antivascular endothelial growth factor treatment on the intratumoral uptake of CPT-11

“…This is remarkable because anti-VEGF mAb clearly decreases tumour VD in this model. These data are consistent with our previous work where we showed a lack of correlation between melphalan tumour uptake and VD (Wildiers et al, 2002). A plausible explanation might be that although fewer vessels are present, they are of better quality, likely allowing improved delivery of bloodborne agents.…”

“…Vessel density was counted on CD34 and CD105 immunostained slides, according to conventional stereological methods using an unbiased Gundersen counting frame (Gundersen, 1978). Briefly, 10 -20 quadrats per tumour were assessed, selected via a stratified random sampling procedure at a magnification of  200, with the aim of counting at least 50 vessels per tumour (Wildiers et al, 2002). The image was unfocused while moving from one quadrat to another to avoid any bias in vessel counting.…”

“…Although VEGF increases vascular permeability, our data do not suggest that blocking VEGF decreases permeability to CPT-11, as delivery was improved rather than diminished. Vascular permeability is probably more important for large molecules (Teicher et al, 1995b), and less so for small molecules (Wildiers et al, 2002) such as CPT-11 and SN-38, which have molecular weights of 586 and 392 g mol À1 , respectively. In short, changes in vessel permeability do not seem to play a major role in the observed increased uptake of CPT-11.…”

Early destruction of tumor vasculature in tumor necrosis factor-α-based isolated limb perfusion is responsible for tumor response

“…This is not seen in sham or melphalan-treated tumors. He also showed that although the transport of melphalan via erythrocytes was less than that in plasma, it correlated well with melphalan availability in embryonic stem cell-derived tumors [27]. Little, however, is known about the contribution of erythrocytes in the transport of melphalan.…”

Impact of Hemoglobin Levels on Tumor Oxygenation: the Higher, the Better?

“…The mechanism(s) by which treatment efficacy and survival are compromised by anemia is (are) not fully understood, but may include an intensification of tumor hypoxia, a more general compromise of the patients' well-being [15,44], poorer transport kinetics of small cytotoxic drugs (i.e., drug delivery) in oligocythemic states [29,65,66,95], or remodeling of tumor microvessels [83]. Whether Hb levels at the start of therapy (or at presentation, e.g., [14,22,25,28,46,62]), at the nadir during therapy (e.g., [26,81]), at the peak during therapy (e.g., [41]), or at the end of (radio)therapy (e.g., [81,94]) are of prognostic value in terms of better disease-free and overall survival is still being assessed in ongoing studies.…”