Sparing radiation-induced damage to the physis by radioprotectant drugs: laboratory analysis in a rat model

Regenerative Medicine Approaches for the Treatment of Pediatric Physeal Injuries

The physis, or growth plate, is a cartilaginous region at the end of children's long bones that serves as the primary center for longitudinal growth and characterizes the immature skeleton. Musculoskeletal injury, including fracture, infection, malignancy, or iatrogenic damage, has risk of physeal damage. Physeal injuries account for 30% of pediatric fractures and may result in impaired bone growth. Once damaged, cartilage tissue within the physis is often replaced by unwanted bony tissue, forming a "bony bar" that can lead to complications such as complete growth arrest, angular or rotational deformities, and altered joint mechanics. Children with a bony bar occupying <50% of the physis usually undergo bony bar resection and insertion of an interpositional material, such as a fat graft, to prevent recurrence and allow the surrounding uninjured physeal tissue to restore longitudinal bone growth. Clinical success for this procedure is <35% and often the bony bar and associated growth impairments return. Children who are not candidates for bony bar resection due to a physeal bar occupying >50% of their physis undergo corrective osteotomy or bone lengthening procedures. These approaches are complex and have variable success rates. As such, there is a critical need for regenerative approaches to not only prevent initial bony bar formation but also regenerate healthy physeal cartilage following injury. This review describes physeal anatomy, mechanisms of physeal injury, and current treatment options with associated limitations. Furthermore, we provide an overview of the current research using cell-based therapies, growth factors, and biomaterials in the different animal models of injury along with strategic directions for modulating intrinsic injury pathways to inhibit bony bar formation and/or promote physeal tissue formation. Pediatric physeal injuries constitute a unique niche within regenerative medicine for which there is a critical need for research to decrease child morbidity related to this injurious process.

Ewing's sarcoma of bone tumor cells produces MCSF that stimulates monocyte proliferation in a novel mouse model of Ewing's sarcoma of bone

Ewing's sarcoma of bone is a primary childhood malignancy of bone that is treated with X-radiation therapy in combination with surgical excision and chemotherapy. To better study Ewing's sarcoma of bone we developed a novel model of primary Ewing's sarcoma of bone and then treated animals with X-radiation therapy. We identified that uncontrolled tumor resulted in lytic bone destruction while X-radiation therapy decreased lytic bone destruction and increased limb-length asymmetry, a common, crippling complication of X-radiation therapy. Osteoclasts were indentified adjacent to the tumor, however, we were unable to detect RANK-ligand in the Ewing's tumor cells in vitro, which lead us to investigate alternate mechanisms for osteoclast formation. Ewing's sarcoma tumor cells and archival Ewing's sarcoma of bone tumor biopsy samples were shown to express MCSF, which could promote osteoclast formation. Increased monocyte numbers were detected in peripheral blood and spleen in animals with untreated Ewing's sarcoma tumor while monocyte number in animals treated with x-radiation had normal numbers of monocytes. Our data suggest that our Ewing's sarcoma of bone model will be useful in the study Ewing's sarcoma tumor progression in parallel with the effects of chemotherapy and X-radiation therapy.

On approaches to the functional restoration of salivary glands damaged by radiation therapy for head and neck cancer, with a review of related aspects of salivary gland morphology and development

Radiation therapy for cancer of the head and neck can devastate the salivary glands and partially devitalize the mandible and maxilla. As a result, saliva production is drastically reduced and its quality adversely altered. Without diligent home and professional care, the teeth are subject to rapid destruction by caries, necessitating extractions with attendant high risk of necrosis of the supporting bone. Innovative techniques in delivery of radiation therapy and administration of drugs that selectively protect normal tissues can reduce significantly the radiation effects on salivary glands. Nonetheless, many patients still suffer severe oral dryness. I review here the functional morphology and development of salivary glands as these relate to approaches to preventing and restoring radiation-induced loss of salivary function. The acinar cells are responsible for most of the fluid and organic material in saliva, while the larger ducts influence the inorganic content. A central theme of this review is the extent to which the several types of epithelial cells in salivary glands may be pluripotential and the circumstances that may influence their ability to replace cells that have been lost or functionally inactivated due to the effects of radiation. The evidence suggests that the highly differentiated cells of the acini and large ducts of mature glands can replace themselves except when the respective pools of available cells are greatly diminished via apoptosis or necrosis owing to severely stressful events. Under the latter circumstances, relatively undifferentiated cells in the intercalated ducts proliferate and redifferentiate as may be required to replenish the depleted pools. It is likely that some, if not many, acinar cells may de-differentiate into intercalated duct-like cells and thus add to the pool of progenitor cells in such situations. If the stress is heavy doses of radiation, however, the result is not only the death of acinar cells, but also a marked decline in functional differentiation and proliferative capacity of all of the surviving cells, including those with progenitor capability. Restoration of gland function, therefore, seems to require increasing the secretory capacity of the surviving cells, or replacing the acinar cells and their progenitors either in the existing gland remnants or with artificial glands.

Radioprotectant combinations spare radiation-induced damage to the physis more than fractionation alone

PURPOSE

The aim of this study was to determine if fractionation and individual or combinations of radioprotectants could minimize damage to physeal longitudinal growth in an animal model to any greater extent than fractionation alone.

MATERIALS AND METHODS

Sixty-three weanling male Sprague-Dawley rats were randomized into seven equal groups. Five groups received a total 25 Gy radiation exposure in three equal fractions to the right knee with the left as non-irradiated control. For each group, pentoxifylline, misoprostol, and amifostine were given individually and amifostine was also given in combination with each of the other drugs prior to the radiation fractions. One group each received 25 Gy in one or three fractions without radioprotection. At six weeks, limb lengths and histomorphometry were assessed.

RESULTS

The single fraction of 25 Gy caused a mean tibial length discrepancy of 24.4%. Fractionation decreased this to 18.8% (p < 0.001). Beyond fractionation alone, the mean femoral length discrepancies were significantly decreased by each of the added individual and combination radioprotectant drugs (p < 0.0004). The smallest absolute femoral length discrepancy (11%) was achieved with fractionation and the combination of amifostine and misoprostol.

CONCLUSIONS

Radioprotectants may be beneficial in growth plate radioprotection, alone or in combination.

Microarray cluster analysis of irradiated growth plate zones following laser microdissection

PURPOSE

Genes and pathways involved in early growth plate chondrocyte recovery after fractionated irradiation were sought as potential targets for selective radiorecovery modulation.

MATERIALS AND METHODS

Three groups of six 5-week male Sprague-Dawley rats underwent fractionated irradiation to the right tibiae over 5 days, totaling 17.5 Gy, and then were killed at 7, 11, and 16 days after the first radiotherapy fraction. The growth plates were collected from the proximal tibiae bilaterally and subsequently underwent laser microdissection to separate reserve, perichondral, proliferative, and hypertrophic zones. Differential gene expression was analyzed between irradiated right and nonirradiated left tibia using RAE230 2.0 GeneChip microarray, compared between zones and time points and subjected to functional pathway cluster analysis with real-time polymerase chain reaction to confirm selected results.

RESULTS

Each zone had a number of pathways showing enrichment after the pattern of hypothesized importance to growth plate recovery, yet few met the strictest criteria. The proliferative and hypertrophic zones showed both the greatest number of genes with a 10-fold right/left change at 7 days after initiation of irradiation and enrichment of the most functional pathways involved in bone, cartilage, matrix, or skeletal development. Six genes confirmed by real-time polymerase chain reaction to have early upregulation included insulin-like growth factor 2, procollagen type I alpha 2, matrix metallopeptidase 9, parathyroid hormone receptor 1, fibromodulin, and aggrecan 1.

CONCLUSIONS

Nine overlapping pathways in the proliferative and hypertrophic zones (skeletal development, ossification, bone remodeling, cartilage development, extracellular matrix structural constituent, proteinaceous extracellular matrix, collagen, extracellular matrix, and extracellular matrix part) may play key roles in early growth plate radiorecovery.

Histomorphometric evidence of growth plate recovery potential after fractionated radiotherapy: an in vivo model

This study evaluated the hypothesis that early growth plate radiorecovery is evident by growth rate, histomorphometric and immunohistochemical end points after exposure to clinically relevant fractionated radiation in vivo. Twenty-four weanling 5-week-old male Sprague-Dawley rats were randomized into eight groups. In each animal, the right distal femur and proximal tibia were exposed to five daily fractions of 3.5 Gy (17.5 Gy) with the left leg serving as a control. Rats were killed humanely at 7, 8, 9, 10, 11, 14, 15 and 16 days after the first day of radiation exposure. Quantitative end points calculated included individual zonal and overall growth plate heights, area matrix fraction, OTC-labeled growth rate, chondrocyte clone volume and numeric density, and BrdU immunohistochemical labeling for proliferative index. Transient postirradiation reductions occurred early and improved during observation for growth rate, proliferative indices, transitional/hypertrophic zone matrix area fraction, proliferative height, and clonal volume. Reserve and hypertrophic zone height remained increased during the period of observation. The current model, using a more clinically relevant fractionation scheme than used previously, shows early evidence of growth plate recovery and provides a model that can be used to correlate temporal changes in RNA and protein expression during the early period of growth plate recovery.

Ontogeny of skeletal maturation in the juvenile rat

Systemic regulation of the cellular processes that produce endochondral elongation and endochondral mineralization during postnatal skeletal maturation are not completely understood. In particular, a mechanism coupling the decline of cellular activity in the bone microenvironment to the onset of sexual maturity remains elusive. The purpose of this study was to empirically integrate the dynamic progression of bone mineral accrual and endochondral elongation as a function of animal age in growing male and female Sprague-Dawley rats. We used serial dual-energy X-ray absorptiometry (DXA) and radiography to study the temporal progression of bone growth and mineral accrual from weaning to adulthood. We observed that skeletal maturation proceeds in a pattern adequately described by the Gompertz function. During this period of growth, we found that serum markers of osteoblastic bone formation declined with age, while osteoclastic bone resorption activity remained unchanged. We also report a slight lag in the age at inflection in the rate of bone mineral accrual relative to the rate of tibial elongation and that both endochondral processes eventually come to asymptotic equilibrium by approximately 20 weeks of age. In addition, we studied tibial growth plate histomorphometry at select time points through 1 year of age. We report that, despite the histologic persistence of physeal cartilage, very little proliferative or elongative activity was measured in this tissue beyond 20 weeks of age. Taken together, these data provide insight to the temporal coordination of postnatal endochondral growth processes.

Radioprotection of craniofacial bone growth

In this review, the potential of pharmacologic therapy for prevention of radiation-induced bone growth inhibition is discussed. Significant radioprotection using the radioprotector Amifostine has been achieved in animal models of radiation-induced retardation of long and craniofacial bone growth. Moreover, radioprotection in vitro has been achieved in a number of cell lines, including osteoblast-like, endothelial, and fibroblastic. This evidence may support future clinical investigations of radioprotector Amifostine or similar substances for radioprotection of the growing craniofacial skeleton.

Restoration of growth plate function following radiotherapy is driven by increased proliferative and synthetic activity of expansions of chondrocytic clones

Radiation therapy encompassing an active epiphysis can negatively impact the potential for bone growth by disrupting cell-cycle progression and accelerating apoptosis and terminal differentiation in physeal chondrocytes. Despite functional derangement following radiation exposure, the irradiated growth plate retains a capacity for regeneration and recovery of growth. The purpose of this study was to characterize the initial sequence of events leading to functional growth recovery in irradiated weanling rat growth plates. We hypothesized that growth in an irradiated epiphysis would be partially restored due to the expansion of chondrocytic clones. Stereological histomorphometry was used to compare chondrocytic cell and matrix turnover between the first and second week following irradiation, and to determine the relative contribution of each of the cellular and extracellular matrix (ECM) compartments to growth. We found that restoration of growth in the irradiated limb was strongly associated with the proliferative activity and production of ECM by these chondrocytic clones, as they expand in average volume, but not in numerical density. We conclude that chondrocytes forming expansive clones and exhibiting increased mitotic and matrix synthesis activity initiate the early restoration of function in the irradiated growth plate, and would be a logical target for strategies to restore full growth potential.

Density and structural changes in the bone of growing rats after weekly alendronate administration with and without a methotrexate challenge

Alendronate (ALN) and other bisphosphonates have been used successfully in pediatric patients with osteopenia secondary to connective tissue diseases. Loss of growth in height has not been reported, but concerns remain regarding the effect of these potent antiresorptive agents when used in children and adolescents. High-dose methotrexate (MTX) and other chemotherapy drugs have been implicated in osteoporosis and a high fracture incidence in survivors of childhood cancers and are also associated with osteopenia in adult animals. The effect of high dose MTX on bone density during rapid skeletal growth, however, has not been widely studied, nor has the potentially therapeutic effect of bisphosphonates in this setting. We examined the effects of ALN and MTX administration, alone and in combination, on bone density, morphology, mechanical strength, and longitudinal growth in normal growing rats. Sprague-Dawley rats were given ALN once weekly (0.3 mg/kg) from 5 to 11 weeks of age, with and without a course of methotrexate (MTX) given daily in weeks 1 and 3 (0.75 mg/kg/day). Twenty-four animals were randomly divided into four groups: Control (vehicle), ALN alone, ALN + MTX, and MTX alone. After 6 weeks, the femora, tibiae, and lumbar spine were studied by dual-energy X-ray absorptiometry, peripheral quantitative computed tomography, mechanical strength testing, microradiography, light microscopy, and by determination of ash weights and bone lengths. ALN treatment increased bone mineral density (BMD) by 23% to 68%. The largest increases in the femur occurred in the distal third where endochondral bone growth was greatest and included large increases in trabecular bone and total cross-sectional area. ALN + MTX produced similar effects to ALN alone. MTX only reduced BMD by 8% in the vertebrae, but not significantly at other sites. MTX also led to femoral length reductions of 2.9%. The small reductions in BMD due to MTX were overwhelmed by the increases due to ALN, whereas the length loss was unaffected. Transverse density banding corresponding to weekly ALN administrations were clearly evident radiographically throughout the growing skeleton, likely due to decreased resorption and possibly increased mineralization in the bands. ALN or ALN + MTX treatment also led to increases in mechanical strength in the femora. Although MTX administration during growth leads to some BMD reduction, ALN given with MTX eliminates this reduction and in fact bone density and strength increase above control levels.

Combination radioprotectors maintain proliferation better than single agents by decreasing early parathyroid hormone-related protein changes after growth plate irradiation

Our hypothesis was that combinations of radioprotectors would be more effective than individual agents in minimizing the effects of radiation on the growth plate after single-fraction hind-limb irradiation of Sprague-Dawley rats. At 2 days postirradiation, the decrease in parathyroid hormone-related protein and parathyroid hormone receptor 1 expression in the irradiated growth plate transitional and hypertrophic zones was reversed in both of the combination groups but persisted in the groups treated with the individual drugs. By 2 weeks, positive findings unique to the combination-treatment animals included greater mean proliferation in the irradiated growth plate than on the contralateral side, smaller limb length discrepancies, reversal of the increased overall matrix area fraction, and reversal of the usual deficiency in Indian hedgehog staining in the irradiated hypertrophic zone. While all treatments had a positive effect in reversing the decrease in B-cell leukemia 2 protein and coincident increase in Bax previously observed 2 weeks postirradiation, the two combination groups had a more robust effect. Combinations of radioprotectors may achieve their beneficial additive effects in the growth plate by decreasing the usual early drop in parathyroid hormone-related protein and parathyroid hormone receptor 1 after irradiation, resulting in a cascade of parathyroid hormone-related protein-mediated events.

Combinations of radioprotectants spare radiation-induced damage to the physis

Radiotherapy used in the treatment of bone and soft tissue sarcomas in pediatric patients often results in undesirable growth plate damage. Radioprotectants may hold promise in the selective protection of growth plate tissue in this setting. In an animal model, the hypothesis tested was that pentoxifylline, selenium, or misoprostol, used in combination with amifostine, would significantly reduce longitudinal growth loss during one radiation dose exposure to a greater extent than the protection provided by only amifostine without increased morbidity or mortality or adverse effects on bone mineral density. Amifostine alone and in combination with each of the other radioprotectants resulted in limb discrepancy reduction to levels significantly less than radiated controls. The tibial length discrepancy in the selenium and amifostine group was 12.1 +/- 0.8%, less than the 15.5 +/- 2.6% tibial length discrepancy in the animals treated with amifostine alone, and less than the mean 18.8% tibial length discrepancy in the radiated limbs without radioprotection. There were no adverse effects on bone density in any group, but the selenium and amifostine group showed some increased mortality. Combinations of amifostine with these radioprotectants show efficacy in growth plate radioprotection and therefore warrant additional study in a clinically relevant fractionated model.

Novel radioprotectant drugs for sparing radiation-induced damage to the physis

PURPOSE

To determine if pentoxifylline, interleukin 1alpha, selenium and misoprostol can minimize damage to physeal longitudinal growth during single radiation dose exposure in an animal model.

MATERIALS AND METHODS

Eighty-seven weanling Sprague-Dawley rats were randomized into 15 drug/dose groups. All groups received a single 17.5-Gy gamma-irradiation exposure to the right knee, the left limb serving as an internal control. Pentoxifylline was injected 30 min before exposure, sodium selenite and interleukin 1alpha 24 h before exposure and misoprostol 2 h before exposure. Positive controls received 17.5 Gy. At 6 weeks, animals were sacrificed, the hind limb lengths were measured and detailed histomorphometric analysis was performed.

RESULTS

Statistically significant reductions (p < or = 0.03) in mean limb length discrepancy compared with irradiation alone were seen following administration of pentoxifylline (50 mg kg(-1)), interleukin 1alpha (15 mcg kg(-1)), selenium (5 mg kg(-1)) and misoprostol (20 mg kg(-1)). Histomorphometric endpoints and growth rate remained altered at 6 weeks despite treatment, but length discrepancy reduction was highly correlated with the appearance of regenerative clones.

CONCLUSIONS

Each drug reduced the amount of anticipated growth arrest in the animal model and some compared favourably in magnitude with that previously demonstrated for the established radioprotectant drug amifostine. Restoration of growth appears related to appearance of regenerative clones.

Decreased proliferation precedes growth factor changes after physeal irradiation

The effects of irradiation on growth plate chondrocytes and mediators of chondrocytic differentiation are poorly understood. In earlier work on rat growth plate changes 1/2 to 4 weeks after irradiation, a nadir was identified at 1 week in proliferation and growth factor expression coincident with maximal histomorphometric derangement. The purpose of this study was to determine the earlier sequential relationship of proliferative, growth factor, and histomorphometric changes after irradiation leading to the 1-week nadir. Twenty-four weanling 5-week-old male Sprague-Dawley rats had right knee irradiation with single fraction 17.5 Gy whereas the left leg served as an internal control. The earliest change identified was a significant decrease in BrdU evidence of proliferative activity between 6 and 12 hours after irradiation, which persisted through 48 hours. Twelve to 24 hours after irradiation, caspase-3 staining for apoptosis was higher than that in growth plates not having received radiotherapy. Histomorphometric changes after irradiation were observed as early as 24 hours. Growth factors and their downstream antiapoptotic and proapoptotic mediators did not differ significantly between limbs through 48 hours. The current study suggests that decreased proliferation and apoptosis precede any change in histomorphometric features of the growth plate after irradiation and that decreased growth factor expression occurs later.

Temporal changes in PTHrP, Bcl-2, Bax, caspase, TGF-beta, and FGF-2 expression following growth plate irradiation with or without radioprotectant

This study examined temporal changes in growth plate apoptosis molecules and growth factors in an animal model of radiation injury with and without a radioprotectant. Thirty weanling 5-week Sprague-Dawley rats underwent right knee irradiation with single-fraction 17.5 Gy while the left served as internal control. Six animals each were sacrificed at 0.5, 1, 2, 3, or 4 weeks after irradiation. Half of the animals received pretreatment with amifostine (WR-2721) radioprotectant. Immunohistochemical staining for PTHrP, Bcl-2, Bax, caspase-3, FGF-2, and TGF-beta was performed. PTHrP decreased to a nadir at 1 week after irradiation but rebounded to above control levels at 2 weeks in the reserve and transitional zones. The radioprotectant amifostine blunted the decrease in PTHrP but kept PTHrP expression lower than controls during the rebound phase in untreated irradiated animals. Hypertrophic zone Bax expression was decreased by amifostine in both irradiated and non-irradiated limbs at 1 and 2 weeks. FGF, TGF-beta, Bcl-2, and caspase levels generally decreased at 1 week and returned thereafter toward control levels. These findings underscore the importance of PTHrP in response to growth plate irradiation and show the novel finding of a decrease in Bax expression with amifostine pretreatment.

Transiently increased bone density after irradiation and the radioprotectant drug amifostine in a rat model

At therapeutic levels in pediatric patients, radiation causes damage to the growth plate and contributes to growth deformity and fractures. The purpose of this project was to examine the effects of x-ray irradiation on regional bone mineral density (BMD) and osteoclast histology of rat bone with and without radioprotectant amifostine (AMF) pretreatment. Seventy-two weanling rats had their right knee irradiated with single fraction 17.5 Gy, whereas the left leg was used as an internal control. Twelve animals were euthanized at each of 6 time periods (0.5-6 wk) after irradiation, half having received 100 mg/kg amifostine. BMD (g/cm3) was determined for both the right and left femurs using peripheral quantitative computed tomography (CT) (pQCT). Tibial sections were stained for osteoclasts/chondroclasts with tartrate-resistant acid phosphatase. Statistically significant increases in BMD within the radiation field were seen in the treatment groups' right irradiated legs over the control unirradiated left legs at all time points from 0.5 through 6 weeks. Anatomically, a peak in BMD occurs in the region immediately adjacent to the chondro-osseous junction at 2 weeks after irradiation and then moves proximally within the adjacent metaphysis after 3 weeks. Corresponding to these findings, histologically a 2-week nadir occurs after irradiation in osteoclasts/chondroclast numbers adjacent to the chondro-osseous junction with a 71.9% decrease compared with controls (p <0.05). At 3 weeks, the numbers of osteoclasts/chondroclasts in this region have increased to 47.4% greater than the control legs (p <0.03) The animals receiving amifostine had BMD that was consistently closer to controls only adjacent to the chondro-osseous junction at 0.5, 2, and 3 weeks and osteoclast/chondroclast numbers that were closer to controls only at 4 weeks.

Sequential histomorphometric analysis of the growth plate following irradiation with and without radioprotection

BACKGROUND

The availability of radioprotectant drugs that selectively protect normal cells but not tumor cells has rekindled interest in the effects of irradiation on the growth plate. The purpose of the present study was to quantitatively examine the sequential histomorphometric effects of irradiation and pretreatment with a free radical scavenger radioprotectant, amifostine, on the growth plate over time.

METHODS

Sixty four-week-old male Sprague-Dawley rats were randomized into five groups of twelve animals that were to be killed at 0.5, one, two, three, or four weeks after irradiation. One-half of the animals also received amifostine (100 mg/kg) prior to irradiation. In all animals, the right knee was treated with a single 17.5-Gy dose of radiation. End points were assessed with quantitative histomorphometric analysis of the growth plate, BrdU labeling for evidence of proliferation, evaluation of chondroclast cellularity, and determination of growth rates by means of oxytetracycline labeling.

RESULTS

The mean lengths of the femur, tibia, and hind limb continued to increase at each time-interval following treatment, but by one week the mean limb length was 4% less on the irradiated side than on the control side, and this difference remained significant for four weeks (p < 0.05). The proximal tibial growth rate decreased during the first week to 18% of the control level. Nevertheless, growth continued even at the earliest time-periods, began to return toward normal at two weeks, and ultimately returned to at least 80% of normal by four weeks after irradiation. The area fraction of matrix in the hypertrophic zone increased initially and returned to control levels at three and four weeks. The administration of the radioprotectant resulted in significant increases in growth, growth rate, growth plate height, hypertrophic zonal height, and chondroclast profiles compared with the values for limbs in which irradiation had not been preceded by treatment with amifostine.

CONCLUSIONS

We found an initially profound but transient direct inhibitory effect of irradiation on growth plate chondrocytes. Recovery of growth plate function after irradiation corresponded temporally with the appearance of newly formed islands of proliferating chondrocytes. Accumulation of matrix led to a transient increase in overall growth plate height, which was most pronounced in the hypertrophic zone. This was due, in part, to the sensitivity of chondroclasts to irradiation. The radioprotectant amifostine reduced these effects on growth rate, growth plate height, matrix accumulation, and limb length.

Efficacy of radioprotection in the prevention of radiation-induced craniofacial bone growth inhibition

It has been reported that radiotherapy-induced craniofacial deformities can occur in 66 to 100 percent of survivors of childhood head and neck cancers. Recent interest in the effectiveness of radioprotectors in the protection of normal tissue against radiation injury led us to investigate a possible role of radioprotection in the prevention of radiation-induced craniofacial bone growth inhibition. Therefore, the objective of this study was to use the radioprotective agent amifostine (Ethyol, WR-2721) as a probe to determine the effectiveness of radioprotection in the prevention of radiation-induced craniofacial bone growth inhibition after single-dose orthovoltage radiation to the infant rabbit orbital-zygomatic complex. Seven-week-old male New Zealand white rabbits were randomized into three groups (n = 10 each): group 1, 0 Gy (sham radiation); group 2, 35-Gy single-dose orthovoltage radiation; and group 3, 35-Gy single-dose orthovoltage radiation and amifostine (300 mg/kg intravenously, given 20 minutes before radiation). Serial radiographs and computed tomographic scans were obtained for cephalometric analysis, bone volume, and bone density measurements until skeletal maturity at 21 weeks. Significant (p < 0.05) reductions in orbital-zygomatic complex linear bone growth, bone volume, and bone density were observed after 35-Gy radiation compared with nonirradiated controls. No significant differences were noted between groups in cephalometric analysis of the nontreated (nonirradiated) left orbital-zygomatic complex, indicating no crossover effect from the radiation beam. However, pretreatment with amifostine, 20 minutes before 35-Gy radiation, resulted in significant (p < 0.05) preservation of linear bone growth, bone volume, and bone mineral density in the rabbit orbital-zygomatic complex compared with controls. This study demonstrated for the first time the effectiveness of a radioprotector in the prevention of radiation-induced craniofacial bone growth inhibition, and it paves the way for investigation into the pathogenic mechanism and prevention of radiotherapy-induced craniofacial deformities.

Amifostine before fractionated irradiation protects bone growth in rats better than fractionation alone

The aim of this study was to determine the independent and combined effects of 100 mg/kg and 200 mg/kg doses of the radioprotectant amifostine and radiotherapy dose fractionation in preserving the integrity of or minimizing damage to the physis during high-dose radiation exposure in an animal model. Thirty-six weanling four-week-old male Sprague-Dawley rats were randomized into six study groups of six animals each. The distal femur and proximal tibia in the right leg of each animal was exposed to X-irradiation, with the contralateral left leg serving as the nonirradiated control. Three groups received a single 25 Gy radiotherapy dose: one group alone, a second group preceded by 100 mg/kg amifostine, and a third preceded by 200 mg/kg amifostine. Three groups received a total of 25 Gy in three equal fractions: one group alone, a second group preceded by 100 mg/kg amifostine, and a third preceded by 200 mg/kg amifostine. Fractionation of the 25 Gy radiation dose reduced the mean percent overall limb growth loss to 44.8%, a statistically significant reduction compared to a mean 58.8% reduced growth with the single 25 Gy dose. Addition of amifostine at 100 and 200 mg/kg before each of the three fractions of radiotherapy further decreased the mean percent overall limb growth loss to 35.2% and 28.5%, respectively, both statistically significant reductions beyond that achieved by fractionation alone.

Sparing of radiation-induced damage to the physis: fractionation alone compared to amifostine pretreatment

PURPOSE

The purpose of this study was to determine the relative benefits of sparing longitudinal bone growth by fractionation alone compared to pretreatment with amifostine, a chemical that provides differential radioprotection of normal tissues.

METHODS AND MATERIALS

Twenty-four weanling 4-week-old male Sprague-Dawley rats were randomized into 2 overall treatment groups: fractionation alone (n = 12) and amifostine pretreatment (n = 12). The distal femur and proximal tibia in the right leg of each animal were exposed to a therapeutic X-irradiation dose (17.5 Gy total in 3 or 5 fractions) with the contralateral left leg as control. In 12 of the animals, amifostine (100 mg/kg) was administered intraperitoneally 20 min before radiation exposure. Six weeks later, growth was calculated based upon measurement of the bone lengths.

RESULTS

Fractionated radiation resulted in a mean percent overall limb growth loss of 21. 1 +/- 7.0%. The addition of amifostine brought the mean percent overall limb growth loss to 16.3% +/- 4.6%, which showed a strong trend toward significance compared to fractionation alone (p = 0. 061). The addition of radioprotection with amifostine to 5 fractions irradiation significantly reduced the femoral and overall percentage growth arrest and limb length discrepancy compared to 5 fractions alone.

CONCLUSIONS

These results support further investigation of amifostine and other radioprotectants in combination with fractionation for use in growing children requiring radiotherapy to the extremity for malignant tumors.