Increased cation transport in mdr1-gene-expressing K562 cells

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

The organic anion transport inhibitor probenecid increases brain concentrations of the NKCC1 inhibitor bumetanide

Bumetanide is increasingly being used for experimental treatment of brain disorders, including neonatal seizures, epilepsy, and autism, because the neuronal Na-K-Cl cotransporter NKCC1, which is inhibited by bumetanide, is implicated in the pathophysiology of such disorders. However, use of bumetanide for treatment of brain disorders is associated with problems, including poor brain penetration and systemic adverse effects such as diuresis, hypokalemic alkalosis, and hearing loss. The poor brain penetration is thought to be related to its high ionization rate and plasma protein binding, which restrict brain entry by passive diffusion, but more recently brain efflux transporters have been involved, too. Multidrug resistance protein 4 (MRP4), organic anion transporter 3 (OAT3) and organic anion transporting polypeptide 2 (OATP2) were suggested to mediate bumetanide brain efflux, but direct proof is lacking. Because MRP4, OAT3, and OATP2 can be inhibited by probenecid, we studied whether this drug alters brain levels of bumetanide in mice. Probenecid (50 mg/kg) significantly increased brain levels of bumetanide up to 3-fold; however, it also increased its plasma levels, so that the brain:plasma ratio (~0.015-0.02) was not altered. Probenecid markedly increased the plasma half-life of bumetanide, indicating reduced elimination of bumetanide most likely by inhibition of OAT-mediated transport of bumetanide in the kidney. However, the diuretic activity of bumetanide was not reduced by probenecid. In conclusion, our study demonstrates that the clinically available drug probenecid can be used to increase brain levels of bumetanide and decrease its elimination, which could have therapeutic potential in the treatment of brain disorders.

Photon activation therapy of RG2 glioma carrying Fischer rats using stable thallium and monochromatic synchrotron radiation

75 RG2 glioma-carrying Fischer rats were treated by photon activation therapy (PAT) with monochromatic synchrotron radiation and stable thallium. Three groups were treated with thallium in combination with radiation at different energy; immediately below and above the thallium K-edge, and at 50 keV. Three control groups were given irradiation only, thallium only, or no treatment at all. For animals receiving thallium in combination with radiation to 15 Gy at 50 keV, the median survival time was 30 days, which was 67% longer than for the untreated controls (p = 0.0020) and 36% longer than for the group treated with radiation alone (not significant). Treatment with thallium and radiation at the higher energy levels were not effective at the given absorbed dose and thallium concentration. In the groups treated at 50 keV and above the K-edge, several animals exhibited extensive and sometimes contra-lateral edema, neuronal death and frank tissue necrosis. No such marked changes were seen in the other groups. The results were discussed with reference to Monte Carlo calculated electron energy spectra and dose enhancement factors.

Physical state changes of membrane lipids in human lung adenocarcinoma A(549) cells and their resistance to cisplatin

The properties of membrane lipids in sensitive A(549) and resistant A(549)/DDP cells to cis-dichlorodiammine platinum[II] (cisplatin) were examined by combining different approaches. The results showed that fluorescence intensity (deltaF) of Merocyanine 540 (MC540) was 93.5 +/- 21.8 for the sensitive A(549) cells and 49.5 +/- 11.2 for the resistive A(549)/DDP cells, monitored by flow cytometry, which may indicate that membrane lipid packing of the sensitive A(549) cells were looser than that of the resistant A(549)/DDP cells. Diffusion rate of N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-1,2-hexadecanoyl-Sn-glycero-3-phosphatidyl-ethanolamine (NBD-PE) was slower in A(549)/DDP cells than in A(549) cells as detected by fluorescence recovery after photobleaching (FRAP) technique. Fatty acid analysis of the membrane lipids showed 21.6, 27.0 and 31.8% increase in the amount of C(18:1), C(18:2) and C(18:3) fatty acid, respectively, in A(549) cells as compared to A(549)/DDP cells. The total amount of unsaturated fatty acids in the plasma membrane lipid is 69.13% +/- 2.2% for A(549), and 55.08% +/- 1.8% for A(549)/DDP cells, respectively. The resistance to cisplatin in A(549)/DDP cells was confirmed by the measurements of the transmembrane influx of Rhodamine-123, cisplatin or Bodipy-cisplatin by fluorescence assay and inductively coupled plasma mass spectrometry (ICP-MS). From the results described previously, it is concluded that changes in the membrane lipids "composition" cause a change in the physical state of the plasma membrane lipids and that this may be associated with the resistance of A(549)/DDP cells to cisplatin.

Relationship between cancer cell proliferation and thallium-201 uptake in lung cancer

Although thallium-201 (201Tl) uptake is related to perfusion in many normal tissues, the biologic rationale for 201Tl uptake in tumors is uncertain. To determine if tumor uptake is related to cell proliferation, we correlated the relative retention of 201Tl in lung tumors with expression of Ki-67, an indicator of cell proliferation.

METHODS

Sixty patients with lung tumors, included small cell carcinoma (n = 8) and non-small cell carcinoma (n = 52), underwent 201Tl single photon emission computed tomography (SPECT) imaging. The 201Tl lesion uptake was determined on early and delayed images and the radiotracer retention index (RI) was calculated. Tumor specimens were obtained at surgery or bronchoscopy. The cell proliferation ratio was estimated with MIB-1, a monoclonal antibody that recognized the nuclear antigen Ki-67.

RESULTS

The average 201Tl index was 2.13+/-0.61 (early) and 2.46+/-0.83 (delayed). The average RI was 17.44+/-35.01. Overall, the 201Tl index (delayed) and the cancer cell proliferation were correlated (r = 0.70, p < 0.0001). Of interest, there was a significant correlation (r = 0.872, p < 0.0005) between the 201Tl index on delayed images and the cell proliferation ratio in patients with small cell but not non-small cell lung carcinoma. The 201Tl index (delayed) was significantly higher (p < 0.0001) in patients with small cell lung carcinoma than in patients with non-small cell lung carcinoma.

CONCLUSION

201Tl imaging appears to be useful for evaluating patients with small cell lung carcinoma but not non-small lung carcinoma, and is correlated with the monoclonal antibody MIB-1, a marker of cell proliferation.