The local anesthetic, n-butyl-p-aminobenzoate, reduces rat sensory neuron excitability by differential actions on fast and slow Na+ current components

Adjunct medications for peripheral and neuraxial anesthesia

Regional and neuraxial anesthesia can provide a safer perioperative experience, greater satisfaction, reduced opioid consumption, and reduction of pain, while minimizing side effects. Ultrasound technology has aided clinicians in depositing local anesthetic medication in precise proximity to targeted peripheral nerves. There are a plethora of adjuvants that have been utilized to prolong local anesthetic actions and enhance effects in peripheral nerve blocks. This manuscript describes the current state of the use of adjuncts, e.g., dexmedetomidine, dexamethasone, clonidine, epinephrine, etc., in regional anesthesia. Additionally, evidence behind dosing and block prolongation is summarized along with patient outcomes, adverse effects, and future directions.

Adjuvants to local anesthetics: Current understanding and future trends

Although beneficial in acute and chronic pain management, the use of local anaesthetics is limited by its duration of action and the dose dependent adverse effects on the cardiac and central nervous system. Adjuvants or additives are often used with local anaesthetics for its synergistic effect by prolonging the duration of sensory-motor block and limiting the cumulative dose requirement of local anaesthetics. The armamentarium of local anesthetic adjuvants have evolved over time from classical opioids to a wide array of drugs spanning several groups and varying mechanisms of action. A large array of opioids ranging from morphine, fentanyl and sufentanyl to hydromorphone, buprenorphine and tramadol has been used with varying success. However, their use has been limited by their adverse effect like respiratory depression, nausea, vomiting and pruritus, especially with its neuraxial use. Epinephrine potentiates the local anesthetics by its antinociceptive properties mediated by alpha-2 adrenoreceptor activation along with its vasoconstrictive properties limiting the systemic absorption of local anesthetics. Alpha 2 adrenoreceptor antagonists like clonidine and dexmedetomidine are one of the most widely used class of local anesthetic adjuvants. Other drugs like steroids (dexamethasone), anti-inflammatory agents (parecoxib and lornoxicam), midazolam, ketamine, magnesium sulfate and neostigmine have also been used with mixed success. The concern regarding the safety profile of these adjuvants is due to its potential neurotoxicity and neurological complications which necessitate further research in this direction. Current research is directed towards a search for agents and techniques which would prolong local anaesthetic action without its deleterious effects. This includes novel approaches like use of charged molecules to produce local anaesthetic action (tonicaine and n butyl tetracaine), new age delivery mechanisms for prolonged bioavailability (liposomal, microspheres and cyclodextrin systems) and further studies with other drugs (adenosine, neuromuscular blockers, dextrans).

The Local Anesthetic Butamben Inhibits and Accelerates Low-Voltage Activated T-Type Currents in Small Sensory Neurons

Butamben (BAB) is a local anesthetic that can be used in epidural suspensions for long-term selective suppression of dorsal root pain signal transmission and in ointments for the treatment of skin pain. Previously, high-voltage activated N-type calcium channel inhibition has been implicated in the analgesic effect of BAB. In the present study we show that low-voltage activated or T-type calcium channels may also contribute to this effect. Typical transient T-type barium currents, selectively evoked by low-voltage (-40 mV) clamp stimulation of small (approximately 20 microm diameter) dorsal root ganglion neurons from newborn mice, were inhibited by BAB with an IC50 value of approximately 200 microM. Furthermore, 200 microM BAB accelerated T-type current activation, deactivation, and inactivation kinetics, comparable to earlier observations for N-type calcium channels. Finally, 200 microM BAB had no effect on the midpoint potential and slope factor of the activation curve, although it caused a approximately 3 mV hyperpolarizing shift of the inactivation curve, without affecting the slope factor. We conclude that BAB inhibits T-type calcium channels with a mechanism associated with channel kinetics acceleration.

The Block of Total and N-Type Calcium Conductance in Mouse Sensory Neurons by the Local Anesthetic n-Butyl-p-Aminobenzoate

To contribute to the understanding of the mechanism underlying selective analgesia by epidural application of suspensions of the local anesthetic butamben (n-butyl-p-aminobenzoate; BAB), we investigated the effect of dissolved BAB on calcium channels in sensory neurons. Small-diameter dorsal root ganglion neurons from newborn mice were used to measure whole-cell barium or calcium currents through calcium channels upon voltage-clamp stimulation. BAB suppressed the voltage-step-evoked barium current of these cells in a concentration-dependent manner with a 50% inhibitory concentration of 207 +/- 14 microM (n = 40). A similar concentration dependency was found for the pharmacologically isolated N-type component of the whole-cell barium current. The time constants of inactivation and deactivation of the N-type current became smaller in the presence of BAB, thus suggesting that kinetic changes are involved in the inhibition of this current. BAB caused a similar inhibition of the total calcium current and its N-type component when these currents were evoked by command potentials with the shape of an action potential. This inhibition of calcium currents by BAB should be considered in the search for the mechanism of selective analgesia by epidural suspensions of this local anesthetic.

Inhibition of the A-type K+ channels of dorsal root ganglion neurons by the long-duration anesthetic butamben

n-Butyl-p-aminobenzoate (BAB; butamben) is a long-duration anesthetic used for the treatment of chronic pain. Epidural administration of BAB is thought to reduce the electrical excitability of dorsal root nociceptor fibers by inhibiting voltage-gated ion channels. To further investigate this mechanism, we examined the effects of BAB on the potassium currents of acutely dissociated neurons from the rat dorsal root ganglion (DRG). These neurons express a rapidly inactivating A-type K(+) current (I(A)) that is resistant to tetraethylammonium (20 mM) but inhibited by 4-aminopyridine (5 mM). At low concentrations, BAB (< or =1 microM) selectively inhibited the I(A) component of DRG K(+) current. The voltage dependence of activation and inactivation, kinetics of recovery from inactivation, and the pharmacology of the DRG I(A) were similar to those of the Kv4 family of K(+) channels. Reverse transcription-polymerase chain reaction was used to establish that the messages encoding for all three of the mammalian Kv4 channel subunits (Kv4.1-Kv4.3) were present in the rat DRG. BAB produced a high-affinity, partial inhibition of heterologously expressed Kv4.2 channels (K(D) = 59 nM) but did not alter the kinetics or voltage sensitivity of gating. Substituting polar threonines for conserved hydrophobic residues of the S6 segment weakened BAB binding but did not alter the voltage-dependent gating of the Kv4.2 channel. At physiological pH, BAB is uncharged, suggesting that hydrophobic interactions may contribute to drug binding. The data support a mechanism in which BAB binds near the narrow cytoplasmic entrance of Kv4 channels and inhibits current by a pore blocking mechanism.

ATP modulation of sodium currents in rat dorsal root ganglion neurons

The modulation of tetrodotoxin-sensitive (TTX-S) and slow tetrodotoxin-resistant (TTX-R) sodium currents in rat dorsal root ganglion neurons by ATP was studied using the whole-cell patch-clamp method. The effects of ATP on two types of sodium currents were either stimulatory or inhibitory depending on the kinetic parameters tested. At a holding potential of -80 mV ATP suppressed TTX-S sodium currents when the depolarizing potential was positive to -30 mV but it increased them when the depolarizing potential was negative to -30 mV. At the same holding potential slow TTX-R sodium currents were always increased by ATP regardless of the depolarizing potential. In both types of sodium currents ATP shifted both the conductance-voltage relationship curve and the steady-state inactivation curve in the hyperpolarizing direction, and accelerated the time-dependent inactivation. ATP decreased the maximum conductance of TTX-S sodium currents but increased that of slow TTX-R sodium currents. The results suggest that ATP would decrease the excitability of neurons with TTX-S sodium channels but would increase that of neurons with slow TTX-R sodium channels. The effects of ATP on sodium currents were preserved in the presence of a G-protein inhibitor, GDP-beta-S, or purinergic antagonists, suramin and Reactive Blue-2, suggesting that purinergic receptors might not be involved in ATP modulation of sodium currents.

Kv1.1 channels of dorsal root ganglion neurons are inhibited by n-butyl-p-aminobenzoate, a promising anesthetic for the treatment of chronic pain

In this study, we investigated the effects of the local anesthetic n-butyl-p-aminobenzoate (BAB) on the delayed rectifier potassium current of cultured dorsal root ganglion (DRG) neurons using the patch-clamp technique. The majority of the K(+) current of small DRG neurons rapidly activates and slowly inactivates at depolarized voltages. BAB inhibited the whole-cell K(+) current of these neurons with an IC(50) value of 228 microM. Dendrotoxin K (DTX(K)), a specific inhibitor of Kv1.1, reduced the DRG K(+) current at +20 mV by 34%, consistent with an important contribution of channels incorporating the Kv1.1 subunit to the delayed rectifier current. To further investigate the mechanism of BAB inhibition, we examined its effect on Kv1.1 channels heterologously expressed in mammalian tsA201 cells. BAB inhibits the Kv1.1 channels with an IC(50) value of 238 microM, similar to what was observed for the native DRG current. BAB accelerates the opening and closing of Kv1.1, but does not alter the midpoint of steady-state activation. BAB seems to inhibit Kv1.1 by stabilizing closed conformations of the channel. Coexpression with the Kv beta 1 subunit induces rapid inactivation and reduces the BAB sensitivity of Kv1.1. Comparison of the heterologously expressed Kv1.1 and native DRG currents indicates that the Kv beta 1 subunit does not modulate the gating of the DTX(K)-sensitive Kv1.1 channels of DRG neurons. Inhibition of the delayed rectifier current of these neurons may contribute to the long-duration anesthesia attained during the epidural administration of BAB.

Involvement of Na+ channels in pain pathways

Voltage-dependent Na+ channels in sensory nerves contribute to the control of membrane excitability and underlie action potential generation. Na+ channel subtypes exhibit a neurone-specific and developmentally regulated pattern of expression, and changes in both channel expression and function are caused by disease. Recent evidence implicates specific roles for Na+ channel subtypes Na(v)1.3 and Na(v)1.8 in pain states that are associated with nerve injury and inflammation, respectively. Insight into the role of Na(v)1.8 in pain pathways has been gained by the generation of a null mutant. Although drugs discriminate poorly between subtypes, the molecular diversity of channels and subtype-specific modulation might provide opportunities to target pain pathways selectively.

Comparison of epidural butamben to celiac plexus neurolytic block for the treatment of the pain of pancreatic cancer

OBJECTIVE

To compare pain relief in metastatic pancreatic cancer patients between neurolytic celiac plexus block (NCPB) and epidural 5% butamben suspension (EBS), a material-based delivery system of a local anesthetic that produces a long-lasting differential nerve block.

DESIGN

Open-label patient-selected parallel groups.

SETTING

Urban tertiary care medical center.

PATIENTS

Twenty-four adult patients with metastatic pancreatic cancer experiencing pain uncontrolled by systemic opioids who were referred to a multidisciplinary pain clinic for interventional therapy.

INTERVENTIONS

Antecrural NCPB-block with ethanol and epidural 5% butamben suspension injections.

MEASURES

Subjective global pain relief assessments on a 0-100% scale were made weekly for 4 weeks and then monthly. Change in opioid use postintervention.

RESULTS

Eight patients had a single NCPB and three patients had two NCPB. Four of the former and two of the latter had successful pain relief defined to be a more than 75% reduction in pain when compared with pretreatment maintained for more than 4 weeks or until death (if less than 4 weeks). Thirteen patients received EBS in divided doses. Eleven patients received a cumulative EBS dose of 5 grams, one patient received a cumulative EBS dose of 2.5 grams, and one patient received a cumulative EBS dose of 8.75 grams. Nine of the eleven patients and each of the other two patients had successful pain relief. The overall incidence (85% EBS vs. 55% NCPB), the duration of successful pain relief, and the percent reduction in opioid use did not differ between the two groups. There were no serious complications.

CONCLUSION

EBS appears to be a safe and effective alternative to NCPB in the treatment of pancreatic cancer pain.

Amitriptyline modulation of Na(+) channels in rat dorsal root ganglion neurons

The effects of amitriptyline, a tricyclic antidepressant, on tetrodotoxin-sensitive and tetrodotoxin-resistant Na(+) currents in rat dorsal root ganglion neurons were studied using the whole-cell patch clamp method. Amitriptyline blocked both types of Na(+)currents in a dose-and holding potential-dependent manner. At the holding potential of -80 mV, the apparent dissociation constants (K(d)) for amitriptyline to block tetrodotoxin-sensitive and tetrodotoxin-resistant Na(+) channels were 4.7 and 105 microM, respectively. These values increased to 181 and 193 microM, respectively, when the membrane was held at a potential negative enough to remove the steady-state inactivation. Amitriptyline dose-dependently shifted the steady-state inactivation curves in the hyperpolarizing direction and increased the values of the slope factors for both types of Na(+) channels. The voltage dependence of the activation of both types of Na(+) channels was shifted in the depolarizing direction. It was concluded that amitriptyline blocked the two types of Na(+) channels in rat sensory neurons by modulating the activation and the inactivation kinetics.

Identification of a yellow impurity in aged samples of aqueous butamben suspension: evidence for the oxidative degradation of poly(ethylene glycol)

Butamben (butyl p-aminobenzoate) has been formulated to provide long-acting treatment for chronic pain. The suspension, which contains poly(ethylene glycol) and polysorbate 80, was found to yellow under ambient conditions if not adequately protected from oxygen. The impurity responsible for the color was isolated and identified on the basis of nuclear magnetic resonance spectroscopy and mass spectrometry. The compound is an oxalamidine, which is formally the condensation product of oxalic acid with four equivalents of butamben, and may be formed by the reaction of butamben with an oxidation product of poly(ethylene glycol).