Drug Metabolizing Enzyme and Transporter Function in Chronic Kidney Disease

Inclusion criteria for CKD patients: * vitamin D deficient (\<30 ng/mL) * hemoglobin \>10 g/dL * willing to abstain from fruit juices or alcohol within 7 days of PK assessments * no changes in prescription or nonprescription medications within 4 wks of study start * age 18-70 yrs * If a diagnosis of CKD, must be due to diabetes mellitus or hypertension * Signed informed consent Inclusion Criteria for Healthy Controls: * vitamin D deficient (\<30 ng/mL) * hemoglobin \>10 g/dL * willing to abstain from fruit juices or alcohol within 7 days of PK assessments * no changes in prescription or nonprescription medications within 4 wks of study start * age 18-70 yrs * Signed informed consent Exclusion criteria for CKD patients: * History of \>14 alcoholic drinks/wk * Not likely to be compliant with study visits * Pregnant or lactating * Predisposition to or history of hypercalcemia * History of allergy, sensitivity, or contraindication to probe drugs (seizures, drug metabolism interactions, etc) * Use of prescribed or nonprescribed therapies that could interact with probe drugs (including prototypical inhibitors or inducers) * Active autoimmune disease or active/recent infections requiring antimicrobial treatment within the previous 4 wks will be excluded to minimize inflammatory-mediated changes in vitamin D status and patient heterogeneity. * Presence of clinically significant hepatic insufficiency (total bilirubin greater than 1.5 times the upper limit of normal or transaminase (ALT, AST) elevations greater than 2 times the upper limit of the laboratory reference range or liver disease * Active seizure disorder or those patients receiving large doses of medications that are known to reduce seizure threshold * Currently receiving cholecalciferol or a vitamin D analogue Exclusion Criteria for Healthy Controls: * History of \>14 alcoholic drinks/wk * Not likely to be compliant with study visits * Pregnant or lactating * Predisposition to or history of hypercalcemia * History of allergy, sensitivity, or contraindication to probe drugs (seizures, drug metabolism interactions, etc) * Use of prescribed or nonprescribed therapies that could interact with probe drugs (including prototypical inhibitors or inducers) * Active autoimmune disease or active/recent infections requiring antimicrobial treatment within the previous 4 wks will be excluded to minimize inflammatory-mediated changes in vitamin D status and patient heterogeneity. * Presence of clinically significant hepatic insufficiency (total bilirubin greater than 1.5 times the upper limit of normal or transaminase (ALT, AST) elevations greater than 2 times the upper limit of the laboratory reference range or liver disease * Active seizure disorder or those patients receiving large doses of medications that are known to reduce seizure threshold * Currently receiving cholecalciferol or a vitamin D analogue * Any clinical evidence of chronic kidney disease as defined by the National Kidney Foundation Dialysis Outcomes Quality Initiative (NKF DOQI) guidelines