Intermittent blood flow in solid tumours--an under-appreciated source of 'drug resistance'.

As the search for improved anti-cancer drugs continues, new paradigms concerning the reasons for clinical failures in common human solid tumours are also evolving. Classical drug resistance is now perhaps less often invoked to explain lack of treatment efficacy than are newer concepts, including 'contact resistance', 'tumour heterogeneity', 'regrowth resistance', and 'physiological barriers' to drug delivery. This commentary will explore the resistance of solid tumours to chemotherapy from yet another, largely ignored perspective: that of tumour-specific fluctuations in blood flow. Transient decreases in blood flow have significant implications for delivery of chemotherapeutic agents, cellular responsiveness to those agents, and the regrowth potential of the surviving tumour cells.