Study to Evaluate the Effects of Zemplar Injection and Calcijex on Intestinal Absorption of Calcium

Inclusion Criteria Subject is ≥ 20 years of age. Subject is diagnosed with ESRD, and must be on maintenance hemodialysis (HD) three times a week for at least 2 months prior to the Screening Phase and expected to remain on HD for the duration of the study. If female, subject is either not of childbearing potential, defined as postmenopausal for at least 1 year or surgically sterile (bilateral tubal ligation, bilateral oophorectomy or hysterectomy), or is of childbearing potential and practicing one of the following methods of birth control: Condoms, sponge, foams, jellies, diaphragm or intrauterine device (IUD) Contraceptives (oral or parenteral) for three months prior to study drug administration In a monogamous relationship with a vasectomized partner If female, subject is not breastfeeding and has a negative serum pregnancy test prior to the treatment phase. Subject had an intact PTH value > 200 pg/mL. Serum calcium level < 10.2 mg/dL at Screening visit. Serum phosphorus level < 6.5 mg/dL at Screening visit. Ca´P product ≤ 65 at Screening visit. Must voluntarily sign and date an informed consent, approved by an Institutional Review Board (IRB)/Independent Ethics Committee (IEC), prior to the conduct of any study-specific procedure Exclusion Criteria Subject has a history of an allergic reaction or significant sensitivity to vitamin D or vitamin D related compounds. Subject has chronic gastrointestinal disease, which in the Investigator's opinion, may result in clinically significant GI malabsorption. Liver function defects defined as > 2 times the upper limit of normal for liver enzyme or > 1.5 times the upper limit of normal coagulation levels. Subject is taking maintenance calcitonin, glucocorticoids in an equivalent dose > 5 mg prednisone, or other drugs that may affect calcium or bone metabolism, other than females on stable (same dose and product for 3 months) estrogen and/or progestin therapy. For any reason, subject is considered by the Investigator to be an unsuitable candidate to receive pharmacological doses of vitamin D. Subject has received any investigational drug within 4 weeks prior to the Treatment Phase.