Passive transport, tumors

Research on the various biochemical systems affected by tumor promoters, in particular TPA, has been recently reviewed by Diamond et al. (54) and Werner and coworkers (55) have reviewed the early effects of phorbol esters on the membranes of cultured cells. The latter group reports that TPA causes permeability changes in 3T3 cell membranes and experimental evidence is cited that phorbol esters interact specifically with a membrane-specific macromolecule rather than passive adsorption by the membrane lipid matrix. One of the earliest observed effects of TPA is a significant modification in the transport of potassium, sodium and phosphate. Lee and Weinstein (56) have found that the addition of phorbol esters immediately stimulated the uptake of 2-deoxyglucose in... 

The nucieobase 5-FU has been extensively used in the treatment of a variety of malignancies however, very little is known about nucieobase transport in mammalian cells. Actually, hENT2 can transport nucleobases, but it is not responsible for 5-FU uptake [90]. Some authors have suggested that 5-FU is taken up by passive diffusion [91] nevertheless, kinetic analysis of 5-FU uptake into tumor cells indicates that it is energy dependent [92, 93]. 

The passive targeting of polymeric micelles to solid tumors can be achieved by the enhanced permeability and retention effect (EPR effect). Maeda and his coworkers presented this new drug targeting strategy in 1986 (31,32). As illustrated in Fig. 3, the vascular permeability of tumor tissues is enhanced by the actions of secreted factors such as kinin. As a result of this increased vascular permeability, macromolecules selectively increase their transport from blood vessels to tumor tissues. Furthermore, the lymphatic drainage system does not operate effectively in tumor tissues. 

Passive targeting occurs due to extravasation of the nanoparticles at the diseased site where the microvasculature is leaky. Passive delivery refers to nanoparticle s transport through leaky tumor capillary fenestrations into the tumor interstitium and cells by passive diffusion or convection (Haley and Frenkel 2008). What is important to understand, passive targeting proceeds based on natural developed... 

It is generally assumed that the inward movement of potassium is passive and is in some way coupled to the expulsion of sodium ions. However, active potassium transport has been observed in the red cell, and tumor cells appear to lose their permeability to potassium and require an active transport mechanism to maintain the intracellular potassium concentration. 

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