Bisphosphonates are pyrophosphate derivatives that bind to the inorganic matrix of bone, and thereby strongly inhibit bone resorption [1,2]. This class of drugs was previously considered for treatment of osteoporosis for conditions in which hormone replacement therapy is restricted. In recent years, however, it has been reported that bisphosphonates have analgesic properties for non-bone-related pains in various preclinical animal models [3-8]. Bonabello et al. [3] showed both a peripherally and centrally mediated analgesic action of 4 bisphosphonates. They used intravenous and intracerebroventricular injection for observing the peripheral and central effect of bisphosphonates and both showed a dose-dependent antinociception. In addition, some biological effects of bisphosphonates are mediated through CNS dendritic cells and microglia [9]. Based on these, the epidural administration of bisphosphonates can be an alternative for the treatment of non-bone-related pain. Currently, bisphosphonates are used clinically by oral administration or intravenous injection. The major disadvantage of orally administrated bisphosphonates is their poor absorption in the gastrointestinal tract [9]. Thus, a relatively high dose of bisphosphonates must be given by intravenous infusion for a rapid and considerable effect. However, intravenous administration causes gastrointestinal disturbances, including nausea, vomiting, and diarrhea, as well as susceptibility to electrolyte imbalance, renal dysfunction, anemia, and hypertension. Moreover, This class of drugs is expensive, and requires hospitalization for long-term intravenous injection [1,2]. Neuroaxial block, such as epidural administration of the drug is expected to reduce dosage, as compared with an oral or intravenous infusion, thereby minimizing systemic adverse effects and maximizing treatment efficacy [10]. This type of administration method has been of great interest in the field of pain medicine. Because it is a non-invasive method, it can be administered long-term and the risk of neurotoxicity is lower.

Although the drug is used safely for systemic administration, it has been well established that animal experiments must be performed to assess efficacy and safety when the drug is administered to the subarachnoid or epidural space [10]. Especially, when the safety is confirmed, we can conduct the preclinical experiment for the efficacy of a drug. Thus, we selected pamidronate, which is one of the most commonly used bisphosphonates for intravenous infusion, for administration into the epidural space in rats. We then performed behavioral observations and histopathologic analysis to assess neurotoxicity.

In the P group, with the exception of 1 rat, and in the N group, no rats showed motor or sensory change, or any behavioral change such as aggressive behavior or crying. Normal reactions to the pinch-toe test included avoidance, crying, and attempts to bite the forceps. All rats in group N and all rats, except 1, in group P had a normal gait at the time point for all examinations (Table 1). During the overall period, however, most rats in Group A showed decreased activity and appetite. In group A, all rats responded with some of the 3 aforementioned reactions, or had no reaction to the pinch-toe test, and had a gait disturbance of Grade 3 or more (P < 0.05, Table 1).

No significantly abnormal histopathologic findings were observed in the P and N groups. However, in group A, various histopathological forms of neurological deficit, such as local neuritis, myelin loss, and meningeal inflammation occurred at each time point of drug administration (P < 0.05, Table 2, Fig. 1, 2).

Bisphosphonates show specific, powerful binding to the inorganic matrix of bone, and thereby suppress the maturation and activity of osteoclasts, ultimately causing apoptosis of osteoclasts [17,18]. One of the bisphosphonates, pamidronate, strongly inhibits bone resorption [1,2]. It is a drug that treats hypercalcemia, osteolytic bone metastasis, and Paget's disease due to malignant tumors by inhibiting bone resorption by osteoclasts.

Pamidronate has been reported to have excellent efficacy in the treatment of chronic lower back pain due to osteoporotic fracture, alleviating pain due to bone metastasis of breast cancer, lowering severe bone pain in Paget's disease, improving the pain of cancer metastasis, and controlling the pain in complex regional pain syndrome [1,19-22].

Bonabello et al. [3] reported that the antinociceptive effects of bisphosphonates occurred with no respect to peripheral opioid receptors in both the central and peripheral nervous systems, with no association in bone lesions. It has also been reported that bisphosphonates modify such substances as prostaglandin E2, sensitizing anti - inerleukine (IL)-1, anti - IL - 6, and anti-tumor necrosis factor - alpha (TNF - alpha) effects, as well as the traumatic pain receptor, and thereby have an analgesic effect, in addition to inhibiting osteolclastic activity [23]. Another study reported that the afferent nerve terminal suppresses the release of various neuropeptides following the onset of trauma in patients with complex regional pain syndrome [22]. In addition, such effects as inhibition of protein prenylation, inhibition of neoangiogenesis, and immune modulation have an influence on the alleviation of pain [24].

Currently, pamidronate is administered via oral route or intravenous infusion. It can be administered up to a maximal dose of 300 mg/d for oral administration and 90 mg/d for intravenous infusion [24]. However, there are a few limitations to administration via the oral route, and disadvantages to the oral administration of bisphosphonate are as follows: 1) a lower rate of intestinal absorption (1-10%) [25]; 2) affection by food ingestion; 3) obligation to administer it during the fasting state; and 4) gastrointestinal disturbances, such as nausea or abdominal pain. Due to these disadvantages, patient compliance has been reported to be lower [25], and injection methods are favored. Intravenous injection has many advantages in terms of cost-effectiveness and patient compliance. A persistent intravenous infusion conversely induces osteochondrosis, and also produces such adverse effects as phlebitis, febrile sensation, chilling sensation, myalgia, malaise, thrombophlebitis, and hypophosphatemia [24,25]. In the early stage of administration, adverse effects, such as increased bone pain, transient leukocytopenia [26], and necrosis of the jaw bone [27,28], used to evolve. In addition, drugs are expensive and hospitalization is needed for an intravenous infusion, which can also be problematic.

From these findings, we considered the epidural treatment of bisphosphonate as an alternative for non-bone-related pain. An epidural approach is currently used most prevalently to control pain, and it is more effective than other administration routes in cases in which the spinal cord is the target of drug administration [29]. Even a minimal dose can produce the same effect, and it is considered that systemic adverse effects and risk of neurotoxicity can be reduced. It is also advantageous in that inhibition of the necessary spinal level can be selective. Pamidronate is also expected to have an excellent effectiveness when directly administered to the area adjacent to the spinal cord. However, administration of any drug into the spinal cord or an epidural space should be evaluated for potential occurrence of neurotoxicity in the spinal cord [10,29]. Although the addictives that can cause neurotoxicity with higher probability, such as anti-oxidants, anti-disinfectants, or excipients, could be ruled out, direct administration of drugs to the area adjacent to the spinal cord can cause direct contact between a high-dose of drugs to the nerve when compared with other administration routes. Therefore, neurological safety must first be demonstrated.

The usual dose of pamidronate recommended for pain control is 15-60 mg/d in humans. In the current study, the dose was 50 times higher than the usual dose for an intravenous infusion based on the weight of the rats, which corresponds to 750 mg in humans. Because 1/30 of the dose for oral administration is administered via epidural route, it is assumed that 22,500 mg is sufficient for a toxicity trial if it is converted to the dosage for oral administration.

Determination of drug volume has been known to correspond with an epidural infusion of 10-15 ml in humans and 0.1 ml in SD rats. In SD rats weighing 250-350 g, a contrast medium of 0.3 ml in volume is distributed across 10-11 segments of the spinal cord. It can therefore be predicted that most drugs with a viscosity lower than that of the contrast medium would be distributed in more extensive areas. It can therefore be inferred that the dosage is sufficient for examination of sensory and motor change in the lower limbs, including the 3-6th lumbar nerve root [30].

Although statistically significant neurotoxicity findings or ambulatory disorder were not observed on light microscopy, an ambulatory disorder was observed in a rat in the P group on day 2 (Grade 4). Because no special abnormal findings were observed on light microscopy, it can therefore be inferred that physical injury caused by catheter use, migration of the catheter into the subarachnoid space, or intravascular migration of the catheter would be a factor responsible for the presence of abnormal findings, rather than drug-induced basic changes in nerves.

In conclusion, epidural administration of pamidronate 12.5 mg/kg did not produce significant neurotoxic adverse effects on behavioral observation and histopathologic examination in SD rats. Further studies in other animal species are warranted to examine the clinical effects of an epidural infusion of pamidronate that could lead to clinical application.