New potential drug targets in ADPKD

Currently there is no good treatment for the most common inherited cause of ESRD, adult polycystic kidney disease. There have been a number of high profile trials in ADPKD in recent years. These trials have endeavored to show a reduction in cyst growth and GFR decline with everolimus, sirolimus) and most recently Tolvaptan (TEMPO). The longer (2years) and larger (433 patients) of the two mTOR inhibitor trials (everolimus) did show a significant reduction in cyst growth at one year but not in GFR reduction. The shorter sirolimus trial failed to show a reduction in cyst growth or GFR decline. The TEMPO trial was over 3 years, had 1445 patients and did show that the V2 antagonist Tolvaptan slowed GFR decline (reciprocal of the serum creatinine level, −2.61 [mg per milliliter]−1 per year vs. −3.81 [mg per milliliter]−1 per year; p=0.001) and cyst growth, 2.8% per year (95% confidence interval [CI], 2.5 to 3.1), versus 5.5% per year in the placebo group (95% CI, 5.1 to 6.0). The jury is still out about the clinical applicability of these drugs and there have been criticisms. For example, tolvaptan is very expensive and would need to be used long term. In the mTOR inhibitor trials some argue doses could have been higher and the lack of hard end points speaks for itself.

However, all is not lost. A potential new drug target in ADPKD was reported by Rowe et al. in Nature Medicine last month. A good overview of the topic can also be found in the same issue.

Using MEF cells from pkd-/- and pkd+/+ mouse littermates they found that growth medium from the pkd-/- cells was more acidic and that the pkd-/- cells had a higher ATP content. To investigate which metabolic pathways might be causing this difference they used NMR spectroscopy and found lower glucose and higher lactate levels in the knock out cells. They then used a mitochondrial ATPase inhibitor to determine the source of higher ATP and found only wt cell had a reduction in ATP with this treatment.  Then the investigators did a real-time PCR analysis on the pkd-/- cells and found an upregulated glycolysis signature. They thus concluded that the pkd-/- cells rely on aerobic glycolysis for their energy demands. This is known as the Warburg effect described in cancer cells (Otto Warburg, a physician-scientist, received the Nobel Prize in Physiology or Medicine in 1931). To see if these in vitro findings translated into in vivo they used Ksp-Cre; Pkd1flox/− mice, which develop early and severe PKD and measured 13C-glucose or 13C-lactate using 13C-NMR. The findings were the same. The authors then used 2-deoxyglucose (2-DG) an analogue of glucose that is unmetabolised. They treated wt and pkd deficient mice this compound and found that the pkd deficient mice had a lower cyst index and lower 13C-glucose consumption as measured using 13C-NMR.

This interesting study proposes that the use of drugs targeting this pathway in combination with other drugs may reduce cystogenesis and progression of CKD in ADPKD. The authors do stress that their summary with regard to human treatments is speculative. In all I think the future is not so gloomy for ADPKD.

Posted by Andrew Malone