12. Glomerular filtration rate (GFR) ≥ 40 mL/min/1.73 m² according to the Modification of Diet in Renal Disease (MDRD) abbreviated formula (within 7 days prior to starting study drug). If not on target, this evaluation may be repeated once after at least 24 hours either according to the MDRD abbreviated formula or by 24 hour sampling. If the later result is within acceptable range, it may be used to fulfill the inclusion criteria instead.