Vitamin D metabolism in the pathogenesis of renal

osteodystrophy and secondary hyperparathyroidism

Geoffrey Block MDDirector of Clinical Research

Denver NephrologistsAssistant Clinical Professor of Medicine

University of Colorado Health Sciences CenterDenver, CO

Kevin Martin MB BChProfessor of Internal Medicine

Director, Division of NephrologySaint Louis University Health Sciences Center

Saint Louis, MO

Vitamin D and hyperparathyroidism

• Vitamin D plays an important role in hyperparathyroidism

• Use of vitamin D metabolites widespread in patients on dialysis

• Potential toxicity of the active metabolites of vitamin D– Use vitamin D analogs instead

• Potential use of vitamin D in earlier stages of kidney disease– Hyperparathyroidism starts very early in the course

of kidney disease

Kevin Martin MB BCh

Calcitriol production

• The active metabolite of vitamin D, calcitriol, is synthesized in the kidney

• Process depends on:– Glomerular filtration – Uptake of 25-OH vitamin D by the proximal

tubule– Delivery to the 1-hydroxylase enzyme in the

mitochondria by binding of the precursor to megalin

Kevin Martin MB BCh

Proteinuria, diabetes, and vitamin D deficiency

• Are patients with proteinuria of modest degree and/or diabetes more susceptible to 25-OH vitamin D deficiency and early development of secondary hyperparathyroidism as a results of their disturbed protein loss in the urine?

Gonzalez EA, Martin KJ et al. Am J Nephrol. 2004;24:503-510.

Geoffrey Block MD

Proteinuria and vitamin D deficiency

• Increased vitamin D deficiency in patients with proteinuria

• Continued loss of vitamin D-binding protein with the bound substrate in the urine Patients with chronic kidney disease

are prone to vitamin D deficiency

Kevin Martin MB BCh

Diabetes and vitamin D deficiency

• Proteinuria-induced deficiency• Other factors seem to be at work in

diabetes: – Diabetics tend not to have severe

hyperparathyroidism in general– Perhaps related to poor glucose control

etc…

Kevin Martin MB BCh

Non-PTH mediated survival effects of vitamin D

• Publication by Drs Thadhani and Teng• Survival advantage of injectable vitamin D in

patients with CKD stage 5 on dialysis • Potential non-PTH-mediated survival effects of

vitamin D• CV mortality rate of people with stage 3 and 4

are increased to levels that cannot be explained by traditional CV risk factors

• Must investigate vitamin D deficiency earlier in stages 3 and 4

Geoffrey Block MD

Teng M, Thadhani R, et al. N Engl J Med. 2003; 349:446-456Teng M, Thadhani R, et al. J Am Soc Nephrol. 2005; 16:1115-1125

Non-PTH mediated survival effects of vitamin D

• Data derived from a retrospective study of patients: hypothesis-generating

• Intriguing observation with potential biological and clinical significance

• Additional research required

Kevin Martin MB BCh

Careful analysis

• Cautious about making broad generalizations of these results

• What is clear is that patients who never get vitamin D appear to be different

• If one is to accept the survival benefits with large amounts of injectable vitamin D then question is: – Whether it should be used in all patients or not

Geoffrey Block MD

Kevin Martin MB BCh

Vitamin D and bone turnover

• Patients without severe hyperparathyroidism should not receive large doses of injectable vitamin D:– Adynamic bone is a risk factor for other

types of cardiovascular disease

“Should we give large doses of vitamin D in the presence of adynamic bone for a presumed

survival benefit and ignore the potential increase in mortality from furthering vascular

calcification?”

Kevin Martin MB BCh

Population differences

• Recent publication by Dr Young:– Analysis using the DOPPS database looking

at effect of mineral metabolism on outcome– In a prevalent population, we’re not able to

see the same profound effect of vitamin D administration as Drs Teng and Thadhani observed

“Population differences play a role in this result”

Geoffrey Block MD

Young EW, Akiba T, et al. Am J Kidney Dis. 2004; 44:34-38

“I prefer moderation”

• Give vitamin D when appropriate without inducing what is overtly oversuppressed parathyroid hormone

• Avoiding high calcium and high phosphorus with or without vitamin D is a required objective

Geoffrey Block MD

Oral vs. injectable vitamin D

• In the DOPS cohort there’s a lot of oral administration of vitamin D vs injectable use:– What is the difference? – What are the postadministration effects?– Are analogs different from calcitriol?– Are all analogs alike? – Can analogs substitute for the active sterols

at other tissue sites?

Kevin Martin MB BCh

Extrarenal effects of vitamin D

• Hard to measure in vivo• Seem to relate to cell differentiation and

gene regulation

Kevin Martin MB BCh

Can we exploit the effect of vitamin D in these tissues in favor of a survival benefit?

Nutritional vitamin D components instead of pharmacological therapy

• Supplements of native vitamin D were given to raise levels of the 25-OH form to above 30 ng/mL

• Sometimes it requires large amounts in this patient population

• We were limited in terms of demonstrating any clinical effects

• May be confounded by the fact that a lot of these patients are also receiving active sterols

Gonzalez EA, Martin KJ et al. Am J Nephrol. 2004;24:503-510.

Kevin Martin MB BCh

Nutritional vitamin D components instead of pharmacological therapy

• Measuring extrarenal effects or “extracalcemic” effects of vitamin D sterols is very difficult

• Replaced by vitamin D2 (ergocalciferol): only available preparation to be given at a reasonably high dose of 50 000 units once a week or once every second week

Gonzalez EA, Martin KJ et al. Am J Nephrol. 2004;24:503-510.

Kevin Martin MB BCh

“With careful study we may see a reduction in the requirement for active sterols but we have

not seen that at the moment.”

Additional questions

• Do those relative deficiencies change in winter vs summer?

• Do the patients’ requirements for 25-OH vitamin D change with the season?

• What are the effects of 25-OH vitamin D repletion in patients who are not receiving large doses of active sterols?

Kevin Martin MB BChGeoffrey Block MD

Substituting with ergocalciferol

• Similar results with stage 3 and 4 with regards to replacing 25-OH vitamin D with ergocalciferol

• You showed a relationship between 25-OH vitamin D and PTH in stages 3 and 4 of CKD

• Replacing with ergocalciferol often is insufficient to achieve K/DOQI targets for parathyroid hormone

Geoffrey Block MD

Differential responses

• In some patients, correcting the 25-OH D deficiency brings PTH levels down to target, whereas in other patients it is not sufficient

• Kidney disease may get so advanced that, although the precursor has been replenished, the remaining kidney tissue can’t produce enough of the active sterol

Kevin Martin MB BCh

Differential responses

• Must categorize the patients:– Diabetics and nondiabetics– Glomerular diseases vs interstitial disease

• Rational approach to treatment rather than give every patient everything

Kevin Martin MB BCh

Concluding remarks

“We’ll evolve towards a broader role of vitamin D beyond hyperparathyroidism”

“Hopefully it will translate into something meaningful in terms of patient

outcome”

Geoffrey Block MD

Kevin Martin MB BCh