Therapeutic use of oral sodium phosphate (phosribbon(®) combination granules) in hereditary hypophosphatemic rickets.

Oral sodium phosphate formulations indicated for hypophosphatemia are commercially available worldwide. In Japan, however, many medical institutes have used hospital dispensary or foreign over-the-counter formulations because no such medication with an indication covered by the health insurance system is domestically available. To address this problem, we initiated the development of Phosribbon(®). The present study evaluated the efficacy and safety of Phosribbon(®) in 16 patients with hereditary hypophosphatemic rickets. The optimal dosage and an administration pattern were also investigated. Administration of the agent resulted in an increase in the level of serum phosphorus in all patients, which implied that the employed dosage was appropriate. The dosage and administration pattern were adjusted based on comprehensive considerations, including changes in clinical laboratory values such as serum phosphorus, alkaline phosphatase and intact PTH, the dosage of a concomitantly administered activated vitamin D formulation and characteristics of individual patients. Adverse drug reactions were observed in 2 patients, neither of which were serious or necessitated therapy dose reduction or discontinuation. We conclude that Phosribbon(®) is a safe and effective treatment for patients with hypophosphatemic rickets and that dose adjustment in this therapy can be guided by the results of regular clinical examination and renal ultrasonography. (ClinicalTrials.gov Identifier: NCT01237288).

Introduction

Hereditary hypophosphatemic rickets is a group of diseases that develop due to congenital impairment of phosphorous reabsorption in proximal renal tubules. The rickets may presumably cause overexpression of fibroblast growth factor 23 (FGF23), a factor that regulates phosphate metabolism, which results in hypophosphatemia, hyperphosphaturia and inhibition of vitamin D activation in the kidney (1,2,3,4,5,6). Therefore, formulations of activated vitamin D and oral sodium phosphate have been employed for the treatment of the rickets (2,3,4,5,6). Although oral sodium phosphate formulations are commercially available in such countries as the United States, the United Kingdom, Germany and France, they have not been covered for hypophosphatemia by the health insurance system in Japan. As a result, many medical institutes in Japan, following review by ethical boards, have employed (a) phosphate reagents for research use; (b) formulations obtained by the pulverization of Visiclear® combination tablets, an oral bowel cleaner containing phosphoric acid; or (c) unapproved drugs after individual import of foreign over-the-counter (OTC) medications (7,8,9).

In order to address this problem, we initiated a development of Phosribbon® combination granules, an oral sodium phosphate formulation. We obtained approval for production and sales of this agent for use in patients with hypophosphatemia in December 2012 from the Japanese Ministry of Health, Labour and Welfare. This report describes the efficacy and safety results associated with the use of Phosribbon® in patients with hereditary hypophosphatemic rickets.

Subjects and Methods

Four medical institutes in Japan performed an open-label, uncontrolled study from September 2010 to December 2011 in an attempt to examine the efficacy and safety of Phosribbon® and to optimize the dosage and administration pattern for it when given orally to patients with hereditary hypophosphatemic rickets. Patients aged 1 to 14 yr were enrolled if they were diagnosed as having hereditary hypophosphatemic rickets based on such things as family history, genetic test results and clinical examination values and a finding of rickets by means of bone X-ray examination. Patients with hyperparathyroidism were excluded.

The study drug contained 330 mg of sodium dihydrogen phosphate monohydrate and 119 mg of anhydrous disodium hydrogen phosphate per envelop (equal to 100 mg of phosphorus). The defined total daily dosage of phosphorus ranged from 300 to 3,000 mg and was to be orally administered 3 or 4 times a day. Patients who had previously received an oral sodium phosphate formulation received the equivalent dosage of phosphorus at the initial administration, while other patients received an initial dose of 30 mg/kg/day of phosphorus 4 times a day (3 times a day if the patient was less than 10.0 kg in weight). The amount of the present formulation was increased in cases where the serum phosphorus value at 1 to 2 h after administration was less than 130% of the value at baseline or in cases where the serum alkaline phosphatase (ALP) level after administration was 150% or more of the level at baseline. Conversely, the drug dose was reduced in cases in which the level of intact parathyroid hormone (PTH) at 1 to 2 h after administration exceeded 130 pg/mL. Investigators were instructed to report reasons for any deviations from the protocol.

Table 1 shows the items of examination, observation, and survey in each period.

Table 1

Items of examination/observation/survey in each period

Efficacy variables

With regard to serum phosphorus, the intervention was considered to be successful when the value for each treatment period (1 to 2 h after administration) was 130% or more of the baseline. For serum ALP, within 150% of the baseline was set as the successful range.

In addition, the frequencies of alterations in dosage and administration patterns were calculated.

A bone X-ray examination was also utilized to compare the findings of rickets between the observation period and each treatment period (wk 24 and 48). The findings were evaluated as either improved, no change or aggravated based on the judgment of physicians.

Safety variables

Safety profiles were investigated using blood and urine tests. In addition, renal ultrasonography was employed to examine renal conditions, such as renal calcification and renal cysts. Furthermore, adverse events and the causal relationship between the interventions were investigated, with adverse events defined as any undesirable or unintended signs, symptoms or diseases that developed when the study drug was administered.

Ethical considerations

The present clinical trial was performed in accordance with ethical principles based on the Declaration of Helsinki and good clinical practice. The parents were well informed of the purpose, and potential risks and benefits of the study using a patient information sheet approved by the institutional review board. Written informed consent was obtained from these parents prior to enrollment. (ClinicalTrials.gov Identifier: NCT01237288)

Results

All 16 patients whose parents provided written informed consent were included in the efficacy and safety analysis sets, and all the patients completed the study. Drug compliance among the 16 patients generally exceeded 80%; but one patient achieved only 50 to 80% compliance with the prescribed treatment in a certain period of time during the treatment period; the patient had trouble remembering to take the drug. Table 2 shows the patient backgrounds. Fifteen out of 16 patients received oral sodium phosphate formulations prior to initiation of the study. All the patients had been treated with activated vitamin D formulations at a dose of 0.037 to 0.129 μg/kg/day. Also, 7 out of 16 patients received GH formulations for the treatment of short stature. FGF23 was determined during the observation period by SRL using an FGF23 ELISA Kit (Kainos Laboratories, Inc., Tokyo, Japan). Its values were over 30 pg/mL in all the patients, which is consistent with the characteristics of hereditary hypophosphatemic rickets defined in an accepted reference source (10).

Table 2

Patient background

Throughout this study, the dosage of Phosribbon® in all the patients ranged from 400 to 2,000 mg/day of phosphorus, which was equivalent to 19.7 to 59.9 mg/kg/day of phosphorus.

Efficacy evaluation

The proportion of patients who experienced successful intervention in terms of the levels of serum phosphorus for each period ranged from 37.5 to 81.3%, and the corresponding serum phosphorus values (mean ± standard deviation) ranged from 3.58 ± 0.70 to 4.07 ± 0.74 mg/dL, which exceeded the baseline of 2.86 ± 0.65 mg/dL. The mean relative values [95% confidence interval] (calculated on the assumption that the value at the observation period was 100%) ranged from 128.52% [114.46, 142.58] to 144.31% [134.89, 153.73] (Fig. 1-A).

Fig. 1.

Mean relative values of (A) serum phosphorus and (B) serum ALP. The value for the observation period was the pre-administration value, and the values for each period were those at 1–2 h post administration.

In regard to serum ALP, the proportion ranged from 93.8 to 100%, and the corresponding serum ALP values ranged from 1,479.4 ± 472.8 to 1,620.1 ± 549.9 U/L, which were almost the same as those at baseline, i.e., 1,554.6 ± 494.3 U/L. The mean relative values ranged from 95.19% [89.68, 100.69] to 106.21% [91.86, 120.55] (Fig. 1-B).

Fifteen patients (59 times in total) met the dose increase and/or dose reduction criteria. Sorted by these criteria, 1 patient (twice in total) met the dose increase criteria for serum ALP, and 14 patients (52 times in total) met the dose increase criteria for serum phosphorus. By contrast, 5 patients (9 times in total) met the dose reduction criteria for intact PTH. Eight patients (13 times in total) experienced changes in dosages and administration patterns; 5 patients (7 times in total) required a dose increase, and 5 patients (6 times in total) required a dose reduction.

Comparison of bone X-ray examination findings of rickets between the observation period and each treatment period (wk 24 and 48) showed that 6 patients experienced improvement and that the remaining 10 patients underwent no change. No aggravation of the disease was observed.

Safety evaluation

Sixteen patients reported adverse events, but none of the patients died or experienced serious adverse events or adverse events requiring discontinuation of the study drug. Table 3 shows the adverse events that developed in 2 or more patients.

Table 3

Adverse events developed in two or more patients

Two out of 16 patients experienced the following adverse drug reactions: 1 patient experienced abdominal pain and allergic dermatitis, and the other experienced diarrhea.

The two gastrointestinal adverse drug reactions (i.e., abdominal pain and diarrhea) were mild in severity and did not necessitate dose reduction or discontinuation of the study drug. Allergic dermatitis was moderate in severity, but the intervention was continued because neither dose reduction nor discontinuation was required; the resultant dermatitis was not aggravated. We failed to deny a causal relation between the intervention and this reaction because the allergic dermatitis developed after initiation of the intervention.

The results of renal ultrasonography showed that 1 patient developed newly reported kidney calcification at wk 48. However, the event was not classified as an adverse event because the kidney calcification was not rapidly aggravated in comparison with the observation period. Some patients reported intact PTH levels of more than 130 pg/mL, but these values were within the range of physiological change, and no patient was diagnosed with hyperparathyroidism.

Discussion

Oral administration of sodium phosphate may result in the improvement of hypophosphatemia, which may lead to remission or improvement in clinical symptoms, as described in several previous reports (2, 3, 11). In Japan, however, no such treatment with an indication for management of hypophosphatemic rickets is covered by the health insurance system, and no integrated therapeutic method has been established.

Therefore, we conducted a study in which Phosribbon® was administered at a predetermined dosage and administration pattern to patients with hereditary hypophosphatemic rickets. Although some decisions made by the investigators deviated from predetermined criteria, we confirmed that administration of Phosribbon® elevated serum phosphorus values. The phosphorus-equivalent dosage of Phosribbon® employed in this study was the same as that described for other formulations in textbooks and in the package inserts of the oral sodium phosphate formulations used in other countries (2, 4,5,6, 12,13,14,15). The dosage and administration patterns were modified based on comprehensive considerations, including changes in serum phosphorus, serum ALP and intact PTH, and the doses of concomitantly administered activated vitamin D formulations, in accordance with the therapeutic guidelines for X-linked hypophosphatemia reported by Carpenter et al. (3). In particular, intact PTH was carefully monitored because its value may affect the safety profile. Also taken into account were the characteristics of individual patients by means of bone X-ray and renal ultrasonography.

As Phosribbon® is an oral sodium phosphate, care must be taken with regard to gastrointestinal disorders, hyperparathyroidism and kidney calcification, although the present study failed to find clinically significant issues related to these conditions.

Marketing approval for Phosribbon® may enable patients suffering from adverse drug reactions to derive benefits from the relief system for sufferers. In addition, the use of this approved formulation will obviate not only the need for the use of non-approved formulations and off-label use but also the associated needs for ethical review, informed consent acquisition, exclusive preparation and individual import. The present data suggest that Phosribbon® is suitable as a common treatment for hypophosphatemia and may be useful as a treatment for hereditary hypophosphatemic rickets.

Conclusions

The present study revealed that Phosribbon® combination granules are a safe and effective treatment for patients with hypophosphatemic rickets and that dose adjustment in this therapy can be guided by results of regular clinical examination and renal ultrasonography.