Pharmacology: Pharmacodynamics: Sodium Alendronate:
Alendronate is a nitrogen-containing bisphosphonate that inhibits osteoclastic bone resorption. It has no direct effect on bone formation. The mode of action postulates incorporation of alendronate present preferentially on the bone resorption surfaces into the cytoplasm of the osteoclast. The activity of osteoclasts is thereby inhibited. After bone resorption stops, bone formation proceeds on the surfaces containing alendronate, which is then incorporated within the matrix where it is no longer pharmacologically active.
The primary pharmacology of alendronate has been investigated in various animal models. Alendronate was administered intermittently, twice weekly (1.8 or 18 mcg/kg SC), to ovariectomised rats in a 1-year treatment study and every 2 weeks (0.05 or 0.25 mg/kg IV) to ovariectomised baboons in a 2-year prevention study. Alendronate prevented or reversed the bone changes produced in the estrogen deficient animals. The changes eg, increased bone turnover, increase in cancellous bone remodeling, decreased bone mineral density (BMD) and bone loss were similar as observed in humans following ovariectomy or menopause.
In other animal models of human bone diseases, alendronate was shown to inhibit bone resorption regardless of cause.
In the long-term study in dogs (36 months at oral doses ranging from 0.25-1 mg/kg/day), alendronate treatment had no deleterious effect on bone quality, bone strength or bone dynamics.
Various animal pharmacodynamic studies show that the action of calcitriol on bone is mediated via the vitamin D receptor (VDR). Because vitamin D3 serves as a prohormone with the little intrinsic biological activity towards VDR, most animal studies have been performed using the fully active hormone 1,25-(OH)2
. In ovariectomised rats, 1,25-(OH)2
increased serum calcium and prevented the ovariectomy-induced lumbar bone loss.
Published data on pharmacodynamic effects of the 1,25-(OH)2
in knockout mice lacking either VDR or the 1α-hydroxylase enzyme confirm the known targets for the active vitamin D3 metabolites. For the 1α-hydroxylase, but not the VDR, knock mice, all bone and cartilage phenotypes were rescued through administration of 1,25-(OH)2
suggesting that 1,25-(OH)2
is the essential metabolite responsible for most biological effects.
Optimum Ratio Study:
In order to derive each optimum concentration ratio of calcitriol and alendronate in combination enabling the bone turnover rate and the serum calcium concentration to be maintained normally while suppressing development of osteoporosis to the maximum extent after menopause by administering calcitriol and alendronate in combination, 1 of calcitriol and alendronate was or both of them were orally administered at 3 different proportions in combination (combination administration group 1, 2 and 3) to female mice for 2 months after ovariectomy was conducted on them. (See Table 1.)
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In all of combination administration groups 1, 2 and 3, the development degree of trabecular bone of tibia was maintained at the same level as in the alendronate-alone administration group. Also, in all of combination administration groups 1, 2 and 3, the serum calcium concentration was maintained at the normal level, which was a good effect. However, in combination administration group 1, the excretory amount of calcium through urine was greatly increased. In combination administration group 3, the excretory amount of calcium through urine was increased, and at the same time, the alkaline phosphatase activity, an osteogenic index was decreased, while in combination administration group 2, the excretory amount of calcium through urine was not increased and the alkaline phosphatase activity was not suppressed.
In comprehensive view of the previously mentioned results, in combination administration group 2 using each concentration ratio of calcitriol and alendronate (calcitriol 0.1 mcg/kg + alendronate 1 mg/kg) in combination, development of osteoporosis was effectively suppressed and the osteogenic velocity was not suppressed, and also, the serum calcium concentration and the excretory amount of calcium through urine were maintained at the normal level. Therefore, it is thought that such concentration ratios are the optimum concentration ratios for development of a pharmaceutical combination.
Mechanism of Action: Sodium Alendronate:
Animal studies have indicated the following mode of action. At the cellular level, alendronate shows preferential localization to sites of bone resorption, specifically under osteoclasts. The osteoclasts adhere normally to the bone surface but lack the ruffled border that is indicative of active resorption. Alendronate does not interfere with osteoclast recruitment or attachment, but it does inhibit osteoclast activity. Studies in mice on the localization of radioactive [3
H]alendronate in bone showed about 10-fold higher uptake on osteoclast surfaces than on osteoblast surfaces. Bones examined 6 and 49 days after [3
H]alendronate administration in rats and mice, respectively, showed that normal bone was formed on top of the alendronate, which was incorporated inside the matrix. While incorporated in bone matrix, alendronate is not pharmacologically active. Thus, alendronate must be continuously administered to suppress osteoclasts on newly formed resorption surfaces. Histomorphometry in baboons and rats showed that alendronate treatment reduces bone turnover (ie, the number of sites at which bone is remodelled). In addition, bone formation exceeds bone resorption at these remodeling sites, leading to progressive gains in bone mass.
Vitamin D3 is produced in the skin by photochemical conversion of 7-dehydrocholesterol to previtamin D3 by ultraviolet light. This is followed by non-enzymatic isomerization to vitamin D3. In the absence of adequate sunlight exposure, vitamin D3 is an essential dietary nutrient. Vitamin D3 in skin and dietary vitamin D3 (absorbed into chylomicrons) is converted to 25-hydroxyvitamin D3 in the liver. Conversion to the active calcium-mobilizing hormone 1,25-dihydroxyvitamin D3 (calcitriol) in the kidney is stimulated by both parathyroid hormone and hypophosphatemia. The principal action of 1,25-dihydroxyvitamin D3 is to increase intestinal absorption of both calcium and phosphate as well as regulate serum calcium, renal calcium and phosphate excretion, bone formation and bone resorption.
Vitamin D is required for normal bone formation. Vitamin D insufficiency develops when both sunlight exposure and dietary intake are inadequate. Insufficiency is associated with negative calcium balance, increased parathyroid hormone levels, bone loss and increased risk of skeletal fracture. In severe cases, deficiency results in more severe hyperparathyroidism, hypophosphatemia, proximal muscle weakness, bone pain and osteomalacia.
The randomized, double-blind, parallel group, comparative, multicenter study was conducted in Korea in 217 postmenopausal women with osteoporosis. Patients were randomized to 2 treatment groups and received once daily over 24 weeks.
The subject were allocated randomly to 1 of the following 2 treatment groups: Test Group: Maxmarvil tablets (alendronate 5 mg + calcitriol 0.5 mcg) + Alfacalcidol placebo tablets.
Reference Group: Maxmarvil placebo tablets + Alfacalcidol tablets.
None of the patients were vitamin-D-deficient, as assessed by serum 25-hydroxyvitamin D [25(OH)D], nor had they received any drugs affecting bone metabolism before enrollment. Bone mineral densities of L1-L4 and the femur were measured by dual-energy x-ray absorptiometry (DXA) at the initial assessment and after 6 months of treatment.
Serum biochemical assays, including serum calcium, 24-hr urinary calcium excretion, and bone turnover markers [both bone-specific alkaline phosphatase (bsALP) and urine N-telopeptide (NTx)], were performed at the baseline, and after 3 and 6 months of treatment.
Primary efficacy endpoint was lumbar spine BMD. The BMD of the lumbar spine increased up to 2.42±5.1% from the baseline after 6 months (p<0.05). On the other hand, the change in BMD in the alfacalcidol group was 0.28±5.01% after 6 months. (See Table 2.)
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The secondary efficacy endpoint was femoral BMD, levels of bsALP and NTx. There was no significant difference in femoral BMD between the 2 groups. The levels of bsALP and NTx were significantly lower in the Maxmarvil group than in the alfacalcidol group [-22.04±39.16% vs -11.42±27.54% (p=0.0219) and -25.46±52.68% vs 1.24±61.5% (p=0.007), respectively]. (See Tables 3 and 4.)
Click on icon to see table/diagram/image
Click on icon to see table/diagram/image
Conclusion on Efficacy:
Study demonstrates that a combination of calcitriol and alendronate is quite effective in preventing bone loss, with the advantage of lesser hypercalciuric effect of calcitriol in the postmenopausal osteoporotic women.
Pharmacokinetics: Absorption: Sodium Alendronate:
As with other bisphosphonates, the oral absorption of alendronate in animals is limited under fasting conditions and negligible in the presence of food. The fasting oral bioavailability of alendronate was estimated as 0.9% in rat, 1.8% in dog and 1.7% in monkey. Oral administration to rats in the presence of food decreases bioavailability about 6- to 7-fold.
Since alendronate is highly polar and charged at physiological pH, absorption across the gastrointestinal tract has been proposed to occur primarily by the paracellular, rather than transcellular route. Alendronate is better absorbed from segments of the GIT with larger surface areas ie, the jejunum > duodenum > ileum.
Calcitriol is rapidly absorbed from the intestine. Peak serum concentrations (above basal values) were reached within 3-6 hrs following oral administration of single doses of calcitriol 0.25-1 mcg. Following a single oral dose of 0.5 mcg, mean serum concentrations of calcitriol rose from a baseline value of 40±4.4 (SD) pg/mL to 60±4.4 pg/mL at 2 hrs, and declined to 53±6.9 at 4 hrs, 50±7 at 8 hrs, 44±4.6 at 12 hrs and 41.5±5.1 at 24 hrs.
Distribution: Sodium Alendronate:
The plasma protein binding (means±SD) determined by ultrafiltration at alendronate concentrations of 0.2-10 mcg/mL (rat, human) and 0.5-5 mcg/mL (dog, monkey) is species-dependent: High to rat plasma proteins (94.76±0.29%), but not to dog (24.6±3%), monkey (68.9±3.8%) or human (72.3±3.7%). At a low concentration range of 0.2-0.5 mcg/mL, the unbound fraction was about 3% for the rat, 23% for the human, 27% for the monkey and 74% for the dog.
Systemically available alendronate disappears very rapidly from plasma, and it is either taken up by bone tissues or excreted by the kidneys. The uptake of alendronate by bone tissues was species-dependent, likely due to plasma flow rate to bone tissues as well as intrinsic bone uptake, and dose-dependent.
Calcitriol is approximately 99.9% bound in blood. Calcitriol and other vitamin D metabolites are transported in blood, by an α-globulin vitamin D binding protein. There is evidence that maternal calcitriol may enter the fetal circulation. Calcitriol is transferred into human milk at low levels (ie, 2.2±0.1 pg/mL).
Metabolism: Sodium Alendronate:
Alendronate is not metabolized in rat and dogs, a finding reported also for other bisphosphonates.
Calcitriol: In vivo
and in vitro
studies indicate the presence of 2 pathways of metabolism for calcitriol. The 1st pathway involves the 24-hydroxylase as the 1st step in catabolism of calcitriol. There is definite evidence of 24-hydroxylase activity in the kidney; this enzyme is also present in many target tissues which possess the vitamin D receptor eg, the intestine. The end-product of this pathway is a side chain shortened metabolite, calcitroic acid. The 2nd pathway involves the conversion of calcitriol via the stepwise hydroxylation of carbon-26 and carbon-23, and cyclization to yield ultimately 1α, 25R(OH)2
-26, 23S-lactone D3
. The lactone appears to be the major metabolite circulating in humans, with mean serum concentrations of 131±17 pg/mL. In addition, several other metabolites of calcitriol have been identified: 1α, 25(OH)2
; 1α, 23,25(OH)3
; 1α, 24R,25(OH)3
; 1α, 25S,26(OH)3
; 1α, 25(OH)2
; 1α, 25R,26(OH)3
; 1α, (OH)24,25,26,27-tetranor-COOH-D3
Excretion: Sodium Alendronate:
Renal excretion is the only route of elimination of alendronate and urinary recovery is quite similar among species. Following IV administration (0.8-1 mg/kg) to rats, dogs and monkeys, approximately 30-40% of the dose was excreted in the urine in 24 hrs, mostly in the first 3-4 hrs. Only a very small fraction, <0.4% of the dose was recovered in rat feces in 24 hrs. Detailed studies in rat revealed that alendronate is actively secreted by an uncharacterized renal transport system, but not by anionic or cationic renal transport systems. Because alendronate is not metabolized and no biliary excretion, compound not excreted in 24 hr post-dose is believed to be sequestered in the skeleton. Once taken up by the bone, the elimination of alendronate from the bone tissue into the circulation to be then eliminated renally is slow, ranging from 200 days in rats to 3 years in dogs.
Enterohepatic recycling and biliary excretion of calcitriol occur. The metabolites of calcitriol are excreted primarily in feces. Following IV administration of radiolabeled calcitriol in normal subjects, approximately 27% and 7% of the radioactivity appeared in the feces and urine, respectively, within 24 hrs. When a 1 mcg oral dose of radiolabeled calcitriol was administered to normal subjects, approximately 10% of the total radioactivity appeared in urine within 24 hrs. Cumulative excretion of radioactivity on the 6th day following IV administration of radiolabeled calcitriol averaged 16% in urine and 49% in feces. The elimination half-life of calcitriol in serum after single oral doses is about 5-8 hrs in normal subjects.
Toxicology: Carcinogenesis, Mutagenesis & Impairment of Fertility: Sodium Alendronate:
Harderian gland (a retroorbital gland not present in humans) adenomas were increased in high-dose female mice (p=0.003) in a 92-week carcinogenicity study at doses of alendronate of 1, 3, and 10 mg/kg/day (males) or 1, 2, and 5 mg/kg/day (females). These doses are equivalent to 0.5-4 times the 10 mg human dose based on surface area (mg/m2
). Parafollicular cell (thyroid) adenomas were increased in high-dose male rats (p=0.003) in a 2-year carcinogenicity study at doses of 1 and 3.75 mg/kg body weight. These doses are equivalent to 1 and 3 times the 10 mg human dose based on surface area.
Alendronate was not genotoxic in the in vitro
microbial mutagenesis assay with and without metabolic activation, in an in vitro
mammalian cell mutagenesis assay, in an in vitro
alkaline elution assay in rat hepatocytes, and in an in vivo
chromosomal aberration assay in mice. In an in vitro
chromosomal aberration assay in Chinese hamster ovary cells, however, alendronate was weakly positive at concentrations 5 mM in the presence of cytotoxicity.
Alendronate had no effect on fertility (male or female) in rats at oral doses up to 5 mg/kg/day (4 times the 10-mg human dose based on surface area).
Long-term studies in animals have not been conducted to evaluate the carcinogenic potential of calcitriol. Calcitriol is not mutagenic in vitro
in the Ames test, nor is it genotoxic in vivo
in the mouse micronucleus test. No significant effects of calcitriol on fertility and/or general reproductive performances were observed in a segment I study in rats at doses of up to 0.3 mcg/kg (approximately 3 times the maximum recommended dose based on body surface area).
Animal Toxicology and/or Pharmacology:
The relative inhibitory activities on bone resorption and mineralization of alendronate and etidronate were compared in the Schenk assay, which is based on histological examination of the epiphyses of growing rats. In this assay, the lowest dose of alendronate that interfered with bone mineralization (leading to osteomalacia) was 6000-fold the antiresorptive dose. The corresponding ratio for etidronate was 1:1. These data suggest that alendronate administered in therapeutic doses is highly unlikely to induce osteomalacia.