COMBINED SEMINAR ON THE USE OF DRUGS IN RENAL FAILURE

Structure-Activity Relationships of Vitamin D Analogues

ALFRED BORIS. Ph.D., Hoffmann-La Roche Inc., Research Division, Nutley, New Jersey

Recent research has shown that vitamin D must un- dergo two metabolic conversions prior to the expression of biologic activity [79-821. A deficiency in the pro- duction of either of these two metabolites can result in clinical manifestations of calcium imbalance and al- terations of bone mineralization. In patients taking an- tiepileptic drugs, rickets or osteomalacia may develop due to interference with the normal pathway for con- version of cholecalciferol to 25-hydroxycholecalciferol [83-851. Administration of the deficient metabolite is effective in the treatment of patients taking anticon- vulsant drugs [86,87]. Hypocalcemia, secondary hy- perparathyroidism and osteodystrophy develop in pa- tients with chronic renal failure because their kidneys are unable to convert 25-hydroxycholecalciferol to la,25dihydroxycholecalciferol. Such patients respond readily to the administration of the missing metabolite, or a synthetic analogue, la-hydroxycholecalciferol [88-911. This new knowledge concerning the metab- olism of vitamin D and the demonstrated clinical utility of the metabolites has stimulated interest in biologic activities of other metabolites and analogues of vitamin D.

Two types of studies were carried out in chicks to assess the biologic effects of nine analogues of vitamin D3. In one study, one day old chicks were fed a D-defi- cient diet [92] and housed under ultraviolet-free lighting for 21 days. Compounds dissolved in propylene glycol were then given orally, and the intestinal absorption of radioactive calcium was determined [93] at intervals following dosing with the secosteroids. This procedure

TABLE VI Onset and Duration of Stimulation of intestinal Calcium Absorption by Metabolites and Analogues of Vitamin D in Chicks

Compound Onset

(hr)

Duration

(hr)

D3 12 144

25(OHID, 12 192 ILU(OH)D, 1.5 96 la,25(OH),D, 1.5 72 24R,25(OH),D, 12 48 24S,25(OH),D, Inactive at 24 hours at 1 ~g dose 11~,24R,25(0H),D, 1.5 24 11~,24825(OH),D, 3 24 5.6.trans D, Inactive at 24 hours at 1 fig dose 5,6-trans 25(OH)D, 24 96

NOTE: One day old male chicks were fed a vitamin D deficient diet for 21 days. A single oral dose of 1 pg of compound was giv- en, and intestinal absorption of ‘%a was determined by the method of Haussler et al [ 151

enabled determination of the onset and duration of ac- tivity for the various compounds. In the second type of study, chicks deprived of dietary vitamin D and ultravi- olet irradiation were given oral doses of compounds once daily for 21 consecutive days. Effects upon body weight, serum calcium and tibia ash weight were measured.

Studies were carried out with vitamin Ds and nine metabolites and analogues to determine the onset and duration of stimulation of intestinal calcium absorption (Table VI). All compounds were given at a dose of 1 pg/chick orally. Intestinal calcium absorption was measured at 24-hour intervals after dosing to determine the duration of activity. Onset of activity was determined by consecutive halving of the time periods from 24 hours. All the tested compounds, except 24S,25(OH)2D3 and 5,6-trans Ds, caused stimulation of intestinal 45Ca absorption 24 hours after a single oral dose of 1 pg/ chick. Both 24S,25(OH)2Ds and 5,6-trans Ds, however, were effective at larger dose levels. Both D3 and 25(OH)Ds stimulated calcium absorption beginning 12 hours after dosing. Significant stimulation was still ev- ident six days later for Ds and eight days after dosing for 25(OH)Ds. Hydroxylation of 25(OH)Ds at the l-alpha position resulted in a compound with a more rapid onset of effect, but also a shorter duration of activity. This compound, the hormonal metabolite of cholecalciferol, lc~,25(OH)~Ds, showed significant stimulation of cal- cium absorption at 90 minutes after dosing, and the activity lasted 72 hours. The synthetic analogue, la(OH)D3, was also effective at 90 minutes, with a

TABLE VII Effects of Some Metabolites and Analogues of Vitamin D, on Tibia Ash Weight in Chicks

50% Increase Dose Relative Compound (nglchicklday) Potency

~... ~~~~ -.__

D3 60 1 .o 25(OH)D, 30 2.0 Icu(O 11 5.5 lo1,25(OH),D, 8 7.5 24R,25(OH),D, 122 0.5 24S,25(OH),D, 1,293 0.05 la,24R,25(OH),D, 73 0.8 la,24S,25(OH),D, 440 0.1 5.6.trans D, 721 0.08 5.6.trans.25(OH)D, 157 0.4 __._ __~__ _.___________~

NOTE: Male chicks were fed a vrtamrn D deficient diet and housed under ultraviolet/free lighting. Beginning at the age of one to two days, compounds were administered orally once daily for 21 consecutive days.

April 1977 The American Journal of Medicine Volume 62 543

COMBINED SEMINAR 0~ THE USA 0~ DRUGS IN RENAL FAILURE

duration of 96 hours. Hydroxylation of 25(OH)Da at po- sition 24 did not change the time of onset of activity but did reduce the duration of response. Further hydroxyl- ation of la,25~(OH)~Ds at position 24 to give the naturally occurring metabolite, la,24R,25(OH)sD3, reduced the duration of activity to 24 hours, suggesting that this compound may represent an excretion product. The trans analogue of 25(OH)D3 took longer to stimulate calcium absorption than 25(OH)Ds, and its duration of effectiveness was shorter than that for 25(OH)Ds.

Daily oral dosing of vitamin D-deficient chicks for 21 consecutive days with 1 to 1,000 ng/chick/day showed that all of the tested metabolites and analogues stimu- lated body weight gain, elevated serum calcium levels and increased tibia ash weights. Tibia ash weight re- sponded to incremental dosage increases in a linear log

dose-response relationship for all compounds (Table VII). Slopes of regression lines were different, however, reflecting differences in absorption, metabolism and excretion, and making exact estimates of relative po- tency difficult. The maximal attainable effect upon tibia ash weight for any compound was an approximate doubling in tibia ash weight above controls. The’ash weight doubling dose for Ds was in excellent .agreement with the minimum daily physiologic requirement for cholecalciferol in the chick of 10 to 15 IU [94]. Com- parison of doses required for a 50 per cent increase in tibia ash weight gave the following order for relative potencies: la,25(OH)*Ds > la(OH)Ds > 25(OH)Ds > D3 > la,24R,25(OH)sD3 > 24R,25(OH)sD3 > 5,6-trans 25(OH)Ds > lcu,24S,25(OH)sDs > 5,6-trans Ds > 24S,25(OH)pDs.

Clinical Studies of Vitamin D Analogues in Renal Failure

DAVID S. DAVID, M.D., Rogosin Kidney Center, The New York Hospital-Cornell Medical Center, New York, New York

Since many of the features of abnormal calcium me- tabolism in renal failure resemble those of vitamin D deficiency, this compound and its related steroids have been used by many physicians in the management of renal osteodystrophy. I will briefly review some of the clinical experience with these drugs and attempt to draw some conclusions as to the vitamin D analogues of choice in the management of renal osteodystrophy. Vitamin D. Pharmacologic doses of vitamin D (D2 or D3) have been demonstrated to increase intestinal ab- sorption of calcium, heal the osteomalacia and osteitis fibrosa, and improve the proximal muscle weakness in patients with renal failure [95-981. The effective dose has varied from 0.1 mg (4,000 IU) to 12.5 mg (500,000 IU) a day [98]. Its therapeutic effect is probably me- diated by its conversion to 25-hydroxycholecalciferol (25 HCC) or some other nonrenal metabolite [98]. Because of the wide variability in its therapeutic dose, its cumulative storage in man and its protracted periods of toxicity [95], few centers today use vitamin D in the management of renal osteodystrophy. Dihydrotachysterol. Studies in uremic patients [99,100] have shown that dihydrotachysterol (DHT) in doses of 0.25 to 0.375 mg/day is effective in stimulating intestinal absorption of calcium and in healing the bony lesions. The therapeutic effects of DHT in uremic pa- tients are most likely mediated by its hepatic hydrox- ylation to 25dihydroxytachysterol (25-DHT) [ 1011. This hepatic metabolite sterically resembles 1,25dihy- droxycholecalciferol (1,25-DHCC) [ 1021, the biologi-

tally active renal hormone of vitamin D [ 1031. It is this sterical resemblance to 1,25-DHCC and the fact that the hepatic 25-hydroxylation of DHT is not feedback- inhibited by 25-DHT [ 1031 that may explain why it is more potent than vitamin D in patients with renal failure. The main advantage of using DHT over vitamin D in uremic patients is its shorter duration of action, which may be related to its lower effective dose. Hypercal- cemia usually responds within a few days to weeks following discontinuation of DHT ([99]; also personal observations). l-Alpha-Hydroxycholecalciferol. Studies with l- alpha-hydroxycholecalciferol (l-a-HCC) in uremic pa- tients reveal that this synthetic analogue of vitamin D can increase the intestinal absorption of calcium, heal the renal osteodystrophy and diminish the proximal muscle weakness in some patients [ 104-1071. Al- though the dose range at which these beneficial effects are produced has varied between 0.5 and 25 pg/day, recent studies [ 104,106] suggest that a dose of 0.5 to 2.0 ,uglday will be sufficient to produce a therapeutic effect in most patients. The effectiveness of this vitamin D analogue in uremic patients-is dependent on the fact that by possessing a hydroxyl group in the l-position, it can bypass the block in renal hydroxylation present in chronic renal failure. Its activity is probably dependent on its hepatic hydroxylation at the 25-position to form 1,25-DHCC [108].

One documented drawback of l-a-HCC, which it shares with vitamin D and DHT [ 1091, is that it is not

544 April 1977 The American Journal of Medicine Volume 62