British ]ournal of Dermatology (1992) 127, 71-78.
REVIEW
Vitamin D analogues and psoriasis J.BERTH-JONES AND P.E.HUTCHINSON Department of Dermatology, Leicester Royal Infirmary. Leicester LEI 5WW, U.K. Accepted for publication 10 April 1992
Summary
Topical vitamin D analogues offer a new, effective, more convenient and generally well-tolerated option for the treatment of psoriasis. Only psoriasis vulgaris has been intensively studied, but other forms ofthe disease may also respond. Both calcitriol and calcipotriol have been shown to be effective in numerous clinical trials, and the latter has compared well with betamethasone valerate and shortcontact dithranol in controlled studies. Their mechanism of action is not yet fully understood and may prove complex. The most important effect may be a direct regulation of keratinocyte proliferation and differentiation. However, these compounds also have potent immuaological properties, and may act by inhibition of cytokine production by keratinocytes or lymphocytes. Topical application of vitamin D analogues appears generally to be remarkably safe, hut hypercalcaemia and hypercalciuria may develop if large quantities are used.
In 1991 the first topical vitamin D analogue (calcipotriol, calcipotriene, 50 /ig/g in an ointment base, Dovonex®, Daivonex®), was licensed in the U.K. for the treatment of psoriasis vulgaris. This review encompasses current evidence regarding the efficacy, mode of action and safety of vitamin D analogues.
Vitamin D and psoriasis Vitamin D became available in commercial quantities in 1930 when vitamin D2 (calciferol, ergocalciferol), was manufactured by irradiation of ergosterol derived from yeast. Another compound found in animal fat became known as vitamin D3, or cholecalciferol. The latter can be produced in the skin by ultraviolet (UVB) irradiation of 7-dehydrocholesterol which is synthesized by the keratinocyte. It has been estimated that an area of skin of just 20 cm^ exposed to the sun for 3 h daily,^ or an oral dose of vitamin D3 of just 10 |ig daily, will prevent rickets. Cholecalciferol requires activation by two hydroxylation reactions catalysed by a hepatic 2 5-hydroxylase, and a renal 1-hydroxylase to produce the active hormone 1,25 dihydroxycholecalciferol (calcitriol). The production of this hormone in the kidney by the final 1hydroxylation is regulated in response to the parathormone level, the plasma phosphate level, and by its own Correspondence: Dr J.Berth-Jones.
level. Excretion of vitamin D and its metabolites is mainly by hepatic secretion into the bile. During the 1930s and 40s vitamin D was prescribed for a very wide variety of diseases. Responses were claimed in conditions as diverse as pemphigus vulgaris,^ scleroderma,^ and tuberculosis.^ Reports appeared of psoriasis responding to large (and toxic) oral doses.^'* A further link between calcium metabolism and psoriasis came from numerous reports of the disease flaring or becoming pustular during episodes of hypocalcaemia. In some cases this hypocalcaemia may have been secondary to the hypoalbuminaemia caused by the severe psoriasis, but in others it was clearly due to malahsorption, or hypoparathyroidism.'"'° These empirical observations were followed by a series of scientific advances. Vitamin D analogues were found to interact with specific high-affinity nuclear receptors." These were identified in many cell types including keratinocytes, dermal fibrohlasts, monocytes and activated lymphocytes.""'* Investigation of these receptors demonstrated that interaction with their ligands promoted keratinocyte differentiation and inhibited proliferation.'^"
Efficacy of vitamin D analogues in psoriasis Interest was stimulated in 1985 by a Japanese report of striking improvement in psoriasis in a patient treated for osteoporosis with oral la-hydroxyvitamin D (alpha71
72
J.BERTH-JONES AND P.E.HUTCHINSON
).''^ These investigators later reported a number of open trials on a total of 52 subjects with psoriasis among whom responses were observed to oral alphacalcidol 1 ^g daily, oral calcitriol 0-5 |ig daily and topical calcitriol 0-1-0-5 ng/g.^^-^° A series of trials performed in the United States involving a total of 124 subjects again demonstrated response of psoriasis to both oral and topical calcitriol. ^^••^' These investigators used the drug systemically at doses starting with 0-25 /ig daily and increasing until hypercalciuria appeared. Topical calcitriol proved effective in placebo-controlled, double-blind studies in ointments containing 3 and 15 /ig/g. In the United Kingdom smaller studies have shown responses to both oral alphacalcidol and topical calcitriol.-^^'^^ Additional placebo-controlled, double-blind studies from Japan^* and Poland^ ^ have confirmed the response to topical calcitriol. Okano^* was able to demonstrate an effect with a concentration as low as 0-1/ig/g. The degree of clinical improvement in these studies was variable, and two double-blind trials showed no response to topical calcitriol."^^'^^ Both of these used a solution of the drug in medium chain triglyceride containing 2 ng/ml. However, taking the results together there can be no doubt that a response occurs, and this is closely related to dose and concentration. In order to minimize effects on calcium metabolism whilst retaining therapeutic activity in psoriasis other vitamin D analogues have been investigated. Topical 1,24 dihydroxycholecalciferol appeared effective in an open study,•^^ but this compound is almost as active as calcitriol in mineral metabolism. Attention has therefore recently focused on calcipotriol, and this has now become the most intensively investigated vitamin D analogue for the treatment of psoriasis. Calcipotriol has less than 1% of the activity of calcitriol on calcium homeostasis.^' The first published study on the use of this compound was a double-blind, placebo-controlled, right/left comparison using a cream formulation.^" Twenty-seven subjects applied vehicle alone to one side ofthe body and an active cream containing one of three concentrations of calcipotriol to the other, twice daily for 6 weeks. There was a clear dose-related improvement on active treatment. In a further trial of similar design, three concentrations in an ointment formulation were compared.^' The response was again significantly better on active treatment than on placebo, and better at 50 than at 25 /ig/g, but did not improve further at 100 /ig/g.
A third placebo-controlled study demonstrated an unequivocal response to topical calcipotriol at 6 weeks using the higher concentration of 1-2 mg/g.^^ In a large, European multicentre study of doubleblind, right/left design, calcipotriol ointment (50 /ig/g) was compared with betamethasone vaierate ointment (0-1%) in 345 subjects." Each treatment was applied twice daily for 6 weeks and response was assessed using the psoriasis area and severity index (PASI). There was a significant difference in favour of calcipotriol at 2, 4 and 6 weeks. A parallel-group, double-blind comparison between calcipotriol and betamethasone vaierate in the same formulations has recently been completed.^* Fourhundred and five patients participated and response was again monitored using the PASI. Both treatments produced a highly significant benefit. There was no significant difference in improvement of PASI between the two treatments, although patients considered calcipotriol more effective. Calcipotriol has been compared with short-contact dithranol therapy in a multicentre, open, parallel-group study.^' Two well-matched groups, each of 239 subjects with chronic plaque psoriasis, were treated for 8 weeks. There was a greater reduction in PASI on calcipotriol than on dithranol, and subjects rated calcipotriol significantly more acceptable than dithranol. A considerable amount of investigation has now been performed relating to long-term use of topical calcipotriol. In the largest long-term study yet published, 167 subjects participated and the duration of treatment was 12 months.^* The mean PASI fell from 8-1 to 2 7 over the period ofthe study. Fifteen subjects were withdrawn due to deterioration of the psoriasis. Fifty complained of some irritation or other cutaneous adverse effect over the 12-month period, but only nine withdrew as a result. No subject developed hypercalcaemia. Research has already begun on combining calcipotriol with other treatments. In a right/left comparison 20 patients used calcipotriol alone on one side of the body and combined with ultraviolet irradiation (UVB) on the other.^'^ Only a small benefit was demonstrated from the addition of UVB. Both calcitriol and calcipotriol have therefore been shown to be effective when used topically in placebocontrolled studies. Calcipotriol is at least as effective as 0-1% betamethasone vaierate or short-contact dithranol therapy, whilst being better tolerated than the latter. In the majority of patients, the beneficial response can be maintained by long-term treatment.
VITAMIN D ANALOGUES AND PSORIASIS
Mechanism of action of vitamin D analogues in psoriasis Vitamin D analogues appear to function in a manner similar to other groups of steroids. They combine with highly specific nuclear receptors which regulate gene transcription by binding, in the presence of a nuclear accessory factor (NAF), to a specific sequence of DNA, known as the vitamin D response element (VDRE).^^ In addition, more immediate 'non-genomic' effects are believed to operate through different mechanisms. These include elevation of intracellular calcium,^' and cyclic GMP levels.*" The presence of the vitamin D receptor in keratinocytes, fibroblasts and immunologically active cells such as macrophages and activated lymphocytes raises a question as to which cell is the principal target through which calcitriol and its analogues act in psoriasis. In vitro studies have demonstrated a potent ability of calcitriol to inhibit cell proliferation and promote differentiation in both mouse'^ and h u m a n " " ' keratinocyte cultures. These properties have also been demonstrated in nunierous other benign and malignant cells lines, including skin fibroblasts,"*^ breast cancer leukaemia cells*^** and histiocytic lymphoma Several other vitamin D analogues,*'"*'' including calcipotriol,^'*^ also demonstrate these actions. Calcitriol may also promote both transcription and release of TGFP by human keratinocytes, and this cytokine suppresses keratinocyte proliferation synergistically with calcitriol.*' The further observation that expression of the vitamin D receptor is up-regulated in hyperproliferative epidermis,'" would strongly support the hypothesis that the keratinocyte is the primary target cell for topical calcitriol analogues in psoriasis. Against this, there are reports that both keratinocytes'' and fibroblasts'^" from psoriatic skin are relatively resistant to inhibition of proliferation by calcitriol, although this seems to vary from patient to patient." Psoriasis is known to respond to cyclosporin, a potent inhibitor of lymphocyte activation. Calcitriol and its analogues share this property, suggesting an immunological mechanism in which the lymphocyte or antigenpresenting cell may be the primary target. The diverse actions of vitamin D analogues include inhibition of lymphocyte activation by interleukin 1 , ' * " inhibition of interleukin 2 production,"""' inhibition of immunoglobulin synthesis,'^'' inhibition of TNF production by monocytes,*" and inhibition of lymphocyte chemotaxis.*' In vivo effects of vitamin D analogues on the skin have
73
been investigated using a variety of methods. In two studies the effect on epidermal proliferation after tape stripping was measured, with confiicting results. In the first no inhibition of proliferation was demonstrated,*^ although the formulation employed was the same as that used unsuccessfully at the same centre to treat psoriasis.^'' In the second, there was marked inhibition of proliferation.*^ Normalization of the epidermal cytokeratin profile during treatment with vitamin D analogues has been demonstrated by various techniques. Keratin electrophoresis on skin biopsies obtained during treatment with oral alphacalcidol demonstrated that expression of keratins K16 and K18, both markers of hyperproliferation, returned to normal levels whilst K2, a marker of differentiation, increased to a level higher than normal.-^-^ Similar changes were observed by the same method during treatment with topical calcipotriol,** and a reduction in K16 expression and DNA content of epidermal cells has been demonstrated by fiow cytometry.*' This evidence would indicate than in vivo, as in vitro, vitamin D analogues inhibit keratinocyte proliferation and promote differentiation, and would therefore suggest that the keratinocyte is a primary target cell. However, histological studies have yielded evidence that immunological changes may also play a role. De Jong et al.^^ found that the earliest change, present at 1 week, was a marked reduction in intensity of polymorph infiltration of the epidermis. This was followed by a reduction in cycling epidermal cells (expressing Ki 67) at 2 weeks, a reduction in epidermal and dermal T lymphocytes by 4 weeks and a relatively small reduction in K16 expression even after 12 weeks. Macrophages fell slightly in both compartments, whilst epidermal Langerhans (16'*') cells increased. Two studies have compared histological changes during treatment with calcipotriol and betamethasone valerate. Mallet et ah" found that histological features were normalized to a similar extent on both treatments, but no reduction was seen in expression of K6 or K16. Berth-Jones et a/.*^ found that K16 was markedly reduced by both treatments, and the effect of betamethasone appeared slightly greater than that of calcipotriol. However, the latter had a more potent effect on K5 and KIO, which were used to measure restoration of normal differentiation. There were also marked reductions in T4 + and T8 + lymphocytes on both treatments. T6 + cells fell rapidly on betamethasone valerate but less so on calcipotriol, and, in some subjects, increased on this treatment. Keratins K5 and KIO each responded to
74
J.BERTH-JONES AND P.E.HUTCHINSON
calcipotriol to a greater degree than epidermal thickness, whereas each component of the infiltrate appeared to 'lag behind'. Expression of IL-6 and TNF-a have been examined using polyclonal antibodies. Calcipotriol reduced keratinocyte expression of IL-6 (a possible regulator of epidermal proliferation), but not TNF-a.''^ These investigations are consistent with a primary effect of vitamin D analogues on keratinocyte proliferation and differentiation. In seems very likely, however, that the immunological properties of these compounds also play a role.
How safe are topical vitamin D analogues? Changes in calcium homeostasis Overdose of vitamin D leads to hypercalcaemia as a result of increased calcium absorption from the gut and resorption of bone. Symptoms which may occur include malaise, headaches, drowsiness, constipation, polydipsia, polyuria, muscle weakness, fatigue, irritability, nausea, and vomiting. Chronic hypercalcaemia may result in urinary stones, soft tissue calcification in blood vessels, myocardium, and cornea, nephrocalcinosis and renal failure. When activated (i.e. 1-hydroxylated) forms of vitamin D such as calcitriol and calcipotriol are used, the physiological control mechanism has been bypassed and there is a risk of hypervitaminosis if use of these compounds is indiscriminate. Conversely, the ability to compensate by reducing endogenous production of calcitriol may provide something of a safety factor. Evidence from the clinical trials described above certainly confirms that oral vitamin D preparations increase serum calcium levels and urinary calcium excretion. Administration of these compounds late at night (when there is less calcium in the bowel), in conjuction with a low calcium diet, may allow larger doses to be given safely.^" The results from topical treatment are more reassuring. In virtually all the clinical trials described above, either serum or urine calcium or both were monitored. With the exception of a case reported below, the only evidence of any disturbance in calcium metabolism during topical treatment with any vitamin D analogue was seen in patients treated with calcitriol by Langner et al.'^^ These investigators found no overall increase in mean serum or urinary calcium during their study. However, among subjects using the highest concentration of calcitriol yet investigated (15 /
REVIEW
Vitamin D analogues and psoriasis J.BERTH-JONES AND P.E.HUTCHINSON Department of Dermatology, Leicester Royal Infirmary. Leicester LEI 5WW, U.K. Accepted for publication 10 April 1992
Summary
Topical vitamin D analogues offer a new, effective, more convenient and generally well-tolerated option for the treatment of psoriasis. Only psoriasis vulgaris has been intensively studied, but other forms ofthe disease may also respond. Both calcitriol and calcipotriol have been shown to be effective in numerous clinical trials, and the latter has compared well with betamethasone valerate and shortcontact dithranol in controlled studies. Their mechanism of action is not yet fully understood and may prove complex. The most important effect may be a direct regulation of keratinocyte proliferation and differentiation. However, these compounds also have potent immuaological properties, and may act by inhibition of cytokine production by keratinocytes or lymphocytes. Topical application of vitamin D analogues appears generally to be remarkably safe, hut hypercalcaemia and hypercalciuria may develop if large quantities are used.
In 1991 the first topical vitamin D analogue (calcipotriol, calcipotriene, 50 /ig/g in an ointment base, Dovonex®, Daivonex®), was licensed in the U.K. for the treatment of psoriasis vulgaris. This review encompasses current evidence regarding the efficacy, mode of action and safety of vitamin D analogues.
Vitamin D and psoriasis Vitamin D became available in commercial quantities in 1930 when vitamin D2 (calciferol, ergocalciferol), was manufactured by irradiation of ergosterol derived from yeast. Another compound found in animal fat became known as vitamin D3, or cholecalciferol. The latter can be produced in the skin by ultraviolet (UVB) irradiation of 7-dehydrocholesterol which is synthesized by the keratinocyte. It has been estimated that an area of skin of just 20 cm^ exposed to the sun for 3 h daily,^ or an oral dose of vitamin D3 of just 10 |ig daily, will prevent rickets. Cholecalciferol requires activation by two hydroxylation reactions catalysed by a hepatic 2 5-hydroxylase, and a renal 1-hydroxylase to produce the active hormone 1,25 dihydroxycholecalciferol (calcitriol). The production of this hormone in the kidney by the final 1hydroxylation is regulated in response to the parathormone level, the plasma phosphate level, and by its own Correspondence: Dr J.Berth-Jones.
level. Excretion of vitamin D and its metabolites is mainly by hepatic secretion into the bile. During the 1930s and 40s vitamin D was prescribed for a very wide variety of diseases. Responses were claimed in conditions as diverse as pemphigus vulgaris,^ scleroderma,^ and tuberculosis.^ Reports appeared of psoriasis responding to large (and toxic) oral doses.^'* A further link between calcium metabolism and psoriasis came from numerous reports of the disease flaring or becoming pustular during episodes of hypocalcaemia. In some cases this hypocalcaemia may have been secondary to the hypoalbuminaemia caused by the severe psoriasis, but in others it was clearly due to malahsorption, or hypoparathyroidism.'"'° These empirical observations were followed by a series of scientific advances. Vitamin D analogues were found to interact with specific high-affinity nuclear receptors." These were identified in many cell types including keratinocytes, dermal fibrohlasts, monocytes and activated lymphocytes.""'* Investigation of these receptors demonstrated that interaction with their ligands promoted keratinocyte differentiation and inhibited proliferation.'^"
Efficacy of vitamin D analogues in psoriasis Interest was stimulated in 1985 by a Japanese report of striking improvement in psoriasis in a patient treated for osteoporosis with oral la-hydroxyvitamin D (alpha71
72
J.BERTH-JONES AND P.E.HUTCHINSON
).''^ These investigators later reported a number of open trials on a total of 52 subjects with psoriasis among whom responses were observed to oral alphacalcidol 1 ^g daily, oral calcitriol 0-5 |ig daily and topical calcitriol 0-1-0-5 ng/g.^^-^° A series of trials performed in the United States involving a total of 124 subjects again demonstrated response of psoriasis to both oral and topical calcitriol. ^^••^' These investigators used the drug systemically at doses starting with 0-25 /ig daily and increasing until hypercalciuria appeared. Topical calcitriol proved effective in placebo-controlled, double-blind studies in ointments containing 3 and 15 /ig/g. In the United Kingdom smaller studies have shown responses to both oral alphacalcidol and topical calcitriol.-^^'^^ Additional placebo-controlled, double-blind studies from Japan^* and Poland^ ^ have confirmed the response to topical calcitriol. Okano^* was able to demonstrate an effect with a concentration as low as 0-1/ig/g. The degree of clinical improvement in these studies was variable, and two double-blind trials showed no response to topical calcitriol."^^'^^ Both of these used a solution of the drug in medium chain triglyceride containing 2 ng/ml. However, taking the results together there can be no doubt that a response occurs, and this is closely related to dose and concentration. In order to minimize effects on calcium metabolism whilst retaining therapeutic activity in psoriasis other vitamin D analogues have been investigated. Topical 1,24 dihydroxycholecalciferol appeared effective in an open study,•^^ but this compound is almost as active as calcitriol in mineral metabolism. Attention has therefore recently focused on calcipotriol, and this has now become the most intensively investigated vitamin D analogue for the treatment of psoriasis. Calcipotriol has less than 1% of the activity of calcitriol on calcium homeostasis.^' The first published study on the use of this compound was a double-blind, placebo-controlled, right/left comparison using a cream formulation.^" Twenty-seven subjects applied vehicle alone to one side ofthe body and an active cream containing one of three concentrations of calcipotriol to the other, twice daily for 6 weeks. There was a clear dose-related improvement on active treatment. In a further trial of similar design, three concentrations in an ointment formulation were compared.^' The response was again significantly better on active treatment than on placebo, and better at 50 than at 25 /ig/g, but did not improve further at 100 /ig/g.
A third placebo-controlled study demonstrated an unequivocal response to topical calcipotriol at 6 weeks using the higher concentration of 1-2 mg/g.^^ In a large, European multicentre study of doubleblind, right/left design, calcipotriol ointment (50 /ig/g) was compared with betamethasone vaierate ointment (0-1%) in 345 subjects." Each treatment was applied twice daily for 6 weeks and response was assessed using the psoriasis area and severity index (PASI). There was a significant difference in favour of calcipotriol at 2, 4 and 6 weeks. A parallel-group, double-blind comparison between calcipotriol and betamethasone vaierate in the same formulations has recently been completed.^* Fourhundred and five patients participated and response was again monitored using the PASI. Both treatments produced a highly significant benefit. There was no significant difference in improvement of PASI between the two treatments, although patients considered calcipotriol more effective. Calcipotriol has been compared with short-contact dithranol therapy in a multicentre, open, parallel-group study.^' Two well-matched groups, each of 239 subjects with chronic plaque psoriasis, were treated for 8 weeks. There was a greater reduction in PASI on calcipotriol than on dithranol, and subjects rated calcipotriol significantly more acceptable than dithranol. A considerable amount of investigation has now been performed relating to long-term use of topical calcipotriol. In the largest long-term study yet published, 167 subjects participated and the duration of treatment was 12 months.^* The mean PASI fell from 8-1 to 2 7 over the period ofthe study. Fifteen subjects were withdrawn due to deterioration of the psoriasis. Fifty complained of some irritation or other cutaneous adverse effect over the 12-month period, but only nine withdrew as a result. No subject developed hypercalcaemia. Research has already begun on combining calcipotriol with other treatments. In a right/left comparison 20 patients used calcipotriol alone on one side of the body and combined with ultraviolet irradiation (UVB) on the other.^'^ Only a small benefit was demonstrated from the addition of UVB. Both calcitriol and calcipotriol have therefore been shown to be effective when used topically in placebocontrolled studies. Calcipotriol is at least as effective as 0-1% betamethasone vaierate or short-contact dithranol therapy, whilst being better tolerated than the latter. In the majority of patients, the beneficial response can be maintained by long-term treatment.
VITAMIN D ANALOGUES AND PSORIASIS
Mechanism of action of vitamin D analogues in psoriasis Vitamin D analogues appear to function in a manner similar to other groups of steroids. They combine with highly specific nuclear receptors which regulate gene transcription by binding, in the presence of a nuclear accessory factor (NAF), to a specific sequence of DNA, known as the vitamin D response element (VDRE).^^ In addition, more immediate 'non-genomic' effects are believed to operate through different mechanisms. These include elevation of intracellular calcium,^' and cyclic GMP levels.*" The presence of the vitamin D receptor in keratinocytes, fibroblasts and immunologically active cells such as macrophages and activated lymphocytes raises a question as to which cell is the principal target through which calcitriol and its analogues act in psoriasis. In vitro studies have demonstrated a potent ability of calcitriol to inhibit cell proliferation and promote differentiation in both mouse'^ and h u m a n " " ' keratinocyte cultures. These properties have also been demonstrated in nunierous other benign and malignant cells lines, including skin fibroblasts,"*^ breast cancer leukaemia cells*^** and histiocytic lymphoma Several other vitamin D analogues,*'"*'' including calcipotriol,^'*^ also demonstrate these actions. Calcitriol may also promote both transcription and release of TGFP by human keratinocytes, and this cytokine suppresses keratinocyte proliferation synergistically with calcitriol.*' The further observation that expression of the vitamin D receptor is up-regulated in hyperproliferative epidermis,'" would strongly support the hypothesis that the keratinocyte is the primary target cell for topical calcitriol analogues in psoriasis. Against this, there are reports that both keratinocytes'' and fibroblasts'^" from psoriatic skin are relatively resistant to inhibition of proliferation by calcitriol, although this seems to vary from patient to patient." Psoriasis is known to respond to cyclosporin, a potent inhibitor of lymphocyte activation. Calcitriol and its analogues share this property, suggesting an immunological mechanism in which the lymphocyte or antigenpresenting cell may be the primary target. The diverse actions of vitamin D analogues include inhibition of lymphocyte activation by interleukin 1 , ' * " inhibition of interleukin 2 production,"""' inhibition of immunoglobulin synthesis,'^'' inhibition of TNF production by monocytes,*" and inhibition of lymphocyte chemotaxis.*' In vivo effects of vitamin D analogues on the skin have
73
been investigated using a variety of methods. In two studies the effect on epidermal proliferation after tape stripping was measured, with confiicting results. In the first no inhibition of proliferation was demonstrated,*^ although the formulation employed was the same as that used unsuccessfully at the same centre to treat psoriasis.^'' In the second, there was marked inhibition of proliferation.*^ Normalization of the epidermal cytokeratin profile during treatment with vitamin D analogues has been demonstrated by various techniques. Keratin electrophoresis on skin biopsies obtained during treatment with oral alphacalcidol demonstrated that expression of keratins K16 and K18, both markers of hyperproliferation, returned to normal levels whilst K2, a marker of differentiation, increased to a level higher than normal.-^-^ Similar changes were observed by the same method during treatment with topical calcipotriol,** and a reduction in K16 expression and DNA content of epidermal cells has been demonstrated by fiow cytometry.*' This evidence would indicate than in vivo, as in vitro, vitamin D analogues inhibit keratinocyte proliferation and promote differentiation, and would therefore suggest that the keratinocyte is a primary target cell. However, histological studies have yielded evidence that immunological changes may also play a role. De Jong et al.^^ found that the earliest change, present at 1 week, was a marked reduction in intensity of polymorph infiltration of the epidermis. This was followed by a reduction in cycling epidermal cells (expressing Ki 67) at 2 weeks, a reduction in epidermal and dermal T lymphocytes by 4 weeks and a relatively small reduction in K16 expression even after 12 weeks. Macrophages fell slightly in both compartments, whilst epidermal Langerhans (16'*') cells increased. Two studies have compared histological changes during treatment with calcipotriol and betamethasone valerate. Mallet et ah" found that histological features were normalized to a similar extent on both treatments, but no reduction was seen in expression of K6 or K16. Berth-Jones et a/.*^ found that K16 was markedly reduced by both treatments, and the effect of betamethasone appeared slightly greater than that of calcipotriol. However, the latter had a more potent effect on K5 and KIO, which were used to measure restoration of normal differentiation. There were also marked reductions in T4 + and T8 + lymphocytes on both treatments. T6 + cells fell rapidly on betamethasone valerate but less so on calcipotriol, and, in some subjects, increased on this treatment. Keratins K5 and KIO each responded to
74
J.BERTH-JONES AND P.E.HUTCHINSON
calcipotriol to a greater degree than epidermal thickness, whereas each component of the infiltrate appeared to 'lag behind'. Expression of IL-6 and TNF-a have been examined using polyclonal antibodies. Calcipotriol reduced keratinocyte expression of IL-6 (a possible regulator of epidermal proliferation), but not TNF-a.''^ These investigations are consistent with a primary effect of vitamin D analogues on keratinocyte proliferation and differentiation. In seems very likely, however, that the immunological properties of these compounds also play a role.
How safe are topical vitamin D analogues? Changes in calcium homeostasis Overdose of vitamin D leads to hypercalcaemia as a result of increased calcium absorption from the gut and resorption of bone. Symptoms which may occur include malaise, headaches, drowsiness, constipation, polydipsia, polyuria, muscle weakness, fatigue, irritability, nausea, and vomiting. Chronic hypercalcaemia may result in urinary stones, soft tissue calcification in blood vessels, myocardium, and cornea, nephrocalcinosis and renal failure. When activated (i.e. 1-hydroxylated) forms of vitamin D such as calcitriol and calcipotriol are used, the physiological control mechanism has been bypassed and there is a risk of hypervitaminosis if use of these compounds is indiscriminate. Conversely, the ability to compensate by reducing endogenous production of calcitriol may provide something of a safety factor. Evidence from the clinical trials described above certainly confirms that oral vitamin D preparations increase serum calcium levels and urinary calcium excretion. Administration of these compounds late at night (when there is less calcium in the bowel), in conjuction with a low calcium diet, may allow larger doses to be given safely.^" The results from topical treatment are more reassuring. In virtually all the clinical trials described above, either serum or urine calcium or both were monitored. With the exception of a case reported below, the only evidence of any disturbance in calcium metabolism during topical treatment with any vitamin D analogue was seen in patients treated with calcitriol by Langner et al.'^^ These investigators found no overall increase in mean serum or urinary calcium during their study. However, among subjects using the highest concentration of calcitriol yet investigated (15 /
