pentazocine transport by square-wave ac iontophoresis with an

So far, pentazocine iontophoresis has neverbeen studied, although pentazocine is widelyused in pain management. The purpose of thisstudy was to determine whether pentazocinetransportation through a cellophane membranecould be enhanced using square-wave alternatingcurrent (AC) iontophoresis with an adjusted dutycycle and dependence on the voltage and the dutycycle. Voltages of 10, 25 and 40 V with duty cyclesof 50%, 51%, 52%, 53%, 54% and 55% wereapplied for 60 minutes at a high frequency of 1 MHzto diffusion cells on both sides of a cellophanemembrane. The donor compartment was filledwith a solution containing pentazocine. Square-wave AC iontophoresis with an adjusted dutycycle enhanced pentazocine transportation athigher voltages and duty cycles. These resultssuggested that the direct current (DC) componentof the square-wave AC played an important role inenhancing pentazocine transportation despitechanges in polarity at very high frequency of1MHz. The higher voltages and duty cyclesinduced a pH change. The practical electrical con-ditions that could be applied clinically were 25 V

with a 54% duty cycle or 40 V with a 53% dutycycle.

Key words: Pentazocine, Iontophoresis, Square-wave, Voltage, Duty cycle

1. Introduction

Pentazocine is a non-narcotic morphine analoguethat is widely used for the management of patients withpostoperative pain or initial carcinogenic pain1,2. Theroutes of pentazocine administration in clinical situa-tions are oral or injection. Because of a high first-passeffect, only 11-32% of an oral dose enters the systemiccirculation in patients3,4. Though the injection of penta-zocine allows a rapid onset of pain relief, the needleinsertion itself is painful and stressful for the patients. Atransdermal route for pentazocine administration maybe a good alternative to avoid these problems.Classical transdermal administration withoutenhancers would be problematic because of the lowpermeability of the skin and the prolonged lag timecaused by the barrier function of the stratumcorneum.

Iontophoresis can be used to enhance transdermaldrug delivery. Local anesthetics, steroids, NSAIDS, opi-oids, peptides, and so on have been administered usingtransdermal drug delivery5,6. The iontophoresis oflocal anesthetics has been used practically in clinicalsituations7-9. Iontophoresis using an AC and DC offset

Original Article

Pentazocine transport by square-wave AC iontophoresis with an adjusted duty cycle

Saori Ogami1, Shizuka Hayashi1, Takao Shibaji2 and Masahiro Umino1

1) Section of Anesthesiology and Clinical Physiology, Department of Oral Restitution, Division of Oral Health Science, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima,Bunkyo-ku, Tokyo, 113-8549, Japan2) Section of Orofacial Pain Management, Department of Oral Restitution, Division of Oral Health Science,Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo, 113-8549, Japan

J Med Dent Sci 2008; 55: 15–27

Corresponding Author: Saori OgamiMailing address: Section of Anesthesiology and Clinical Physiology,Department of Oral Restitution, Division of Oral Health Science,Graduate School, Tokyo Medical and Dental University, 1-5-45Yushima, Bunkyo-ku, Tokyo, 113-8549, Japan Tel: +81-3-5803-5552 Fax: +81-3-5803-0206 E-mail address: [email protected] September 6; Accepted November 30, 2007

has also been used for the treatment of patients withhyperhidrosis10. For example, an iontophoretic trans-dermal system of fentanyl hydrochloride, which ispatient-activated, has been used in humans11. Thesymmetrical nature of iontophoresis, the nature thations are driven both into and out of the body, has alsobeen utilized to extract data from the body without bloodsampling. A reverse iontophoresis device has alreadybeen introduced for glucose monitoring in patientswith diabetes12. Thus, iontophoretic transdermal deliv-ery is very useful for improving the quality of life (QOL) of patients. The iontophoresis of pentazocine has notyet been studied (not even using DC iontophoresis).Narcotics such as morphine and fentanyl are strictlyregulated during clinical use. Pentazocine, which isnon-narcotic analgesics, is easier to use than narcoticanalgesics.

Standard iontophoresis employs a continuous DC.Continuous DC iontophoresis has some adverseeffects, including electrochemical burns, erythema,and a reduced transportation effect as a result of thepolarization of the skin and the electrode13,14. In aneffort to avoid these problems, AC and pulsed DC havealso been applied for iontophoretic drug delivery. ACiontophoresis has the advantage of not causing elec-trochemical burns because the polarity of the currentalternates periodically. The optimal conditions for cur-rent application, including the waveform, the amplitude,the voltage, and the frequency, have been investigatedfor pulsed DC iontophoresis, DC iontophoresis withalternating polarity, and AC iontophoresis with DC off-set10,15-17,18-22. Though DC iontophoresis is more effec-tive than AC iontophoresis for the transportation ofdrugs, the duration of DC iontophoresis is limited to 10-to-15-minute periods because of the electrochemicalburns produced by the hydrogen and hydroxide ionsthat are generated by the DC current13. On the otherhand, AC iontophoresis enables a long duration of cur-rent application because of minimal polarization of theskin and electrode and minimal skin irritation.

In this study, a square-wave with an adjusted dutycycle of polarity alternation was employed to balancethe advantages of AC and DC iontophoresis for penta-zocine transportation. We determined influence ofvoltage and duty cycle on transport efficiency of pen-tazocine.

The focus of this research was to investigatewhether pentazocine can be transported efficientlyusing square-wave AC iontophoresis with an adjustedduty cycle and to find the optimal duty cycle and voltagefor pentazocine transportation.

2. Materials and Methods

2.1. Materials Pentazocine (PENTAGIN� injection) was purchased

from SANKYO Co., Ltd. (Tokyo, Japan). This commer-cial injection contains pentazocine (30 mg), lactic acid(12 ÒL), and sodium chloride (2.8 mg) in a total volumeof 1 mL. The pentazocine injection was diluted with dis-tilled water to adjust the pentazocine concentration to0.5 mg/mL (pH 4.4).

2.2. MembraneThe cellophane membrane was about 36 Òm thick

with pore sizes of about 2-3 nm; these pore sizes wereone order of magnitude larger than the size of the ionsused in this study.

2.3. Permeation experimentsThe cellophane membrane was placed between a

pair of acrylic diffusion cells with diameters of 15 mmand length of each compartment was 10 mm.Platinum plate electrodes (99.95% purity), with adiameter of 15 mm and a thickness of 0.15 mm wereinstalled at opposite ends of the two compartments ofthe diffusion cells, as seen in Fig. 1. The donor com-partment was filled with 2 mL of solution containingpentazocine (0.5 mg/mL), and the receptor compart-ment was filled with 2 mL of distilled water (Fig. 1). Thediffusion cells were set in a water bath, and the tem-perature in the receptor compartment was controlled soas not to exceed 37°C.

A temperature probe (Model BAT-12; Physitemp,USA) was inserted at the center of the receptor com-

S. OGAMI et al. J Med Dent Sci16

Fig. 1. Diagram of the experimental system. The experimental sys-tem consisted of two diffusion cells, a temperature probe, a waterbath, a function generator and a high-speed amplifier.

partment to monitor the temperature of fluid in it. A 20-ÒL sample was taken from the center of the receptorcompartment every 15minutes during the application ofthe electrical current. All experiments were replicatedfive times. The solution in the receptor compartmentwas not replaced because the amount of the sample,20 ÒL, was negligibly small, compared with the volumeof 2 mL in the receptor compartment. The sampleswere analyzed using a high performance liquid chro-matography (HPLC) system. The pH in both compart-ments was measured using a pH meter (pHBOY-P2;Shindengen Electric MFG. Co., Ltd, Japan) after sam-pling. The solutions in both compartments were notstirred to avoid serious effects on diffusion.

The electric field was applied using a function gen-erator (Model number 8116A; Hewlett Packard, Tokyo,Japan) and a high speed power amplifier (Model num-ber 4025; NF Electric Instruments, Kanagawa,Japan). The waveform was monitored using an oscillo-scope (Model number 54503A; Hewlett Packard,Tokyo, Japan). A square-wave AC with duty cycles of50%, 51%, 52%, 53%, 54% or 55% was applied for 60minutes. The waveform from function generator with aduty cycle of A% is shown in Fig. 2. The waveformthrough the cellophane membrane was slightlydeformed at high frequencies because of the capaci-tance and inductance of the circuit.

Three different voltages, 10 V, 25 V and 40 V wereapplied for each duty cycle condition. The frequency ofthe applied electric field was kept at 1 MHz. The volt-ages and frequency were selected based on the

results of previous studies24-26.

2.4. Drug analysis The concentrations of pentazocine was determined

using an HPLC system (Shimadzu, Japan) with anappropriate column (Shim-pack� VP-ODS, 150 mm×4.6 mm, Shimadzu, Japan) and a mobile phase com-posed of 10mM phosphate buffer (pH 2.6): acetonitrile(75:25) at a flow rate of 0.8 mL/min. The column wasmaintained at a temperature of 40°C. UV detection wasperformed at a wavelength of 278 nm. The flux(nmol/hr/cm2) of pentazocine was calculated from thecumulative amount of pentazocine transported to thereceptor compartment over a period of 60 minutes.

The lactic acid concentration in the donor compart-ment after 60 minutes of iontophoresis was determinedusing an HPLC system with the same column as thatused for pentazocine and a mobile phase composed of10 mM phosphate buffer (pH 2.6). UV detection wasperformed at a wavelength of 210 nm.

2.5. Statistical analysisAll experiments but transportation of lactic acid

were replicated five times; the results were expressedas the mean±standard error (S.E.). Statistical analyseswere performed by means of Microsoft Excel 2003.Simple linear regression analysis was used to examinethe relationship between time and pentazocine con-centration, the relationship between pentazocine fluxesand predictor variables (duty cycle or applied voltage)the relationship between lactic acid concentration andduty cycle and the relationship between pH and pre-dictor variables (duty cycle or applied voltage).Coefficient of determinations (R2) were calculatedusing the least-squares method. P-values to the slopeof <0.05 were regarded as statistically significant.

3. Results

3.1. The time courses for the transport of penta-zocine

Figure 3 shows the time courses for the transport ofpentazocine from the donor compartment to thereceptor compartment under the application of 10 V, 25V and 40 V with a 55% duty cycle at 1 MHz and underpassive diffusion for 60 minutes. The pentazocineconcentration in the receptor compartment increased ina time-dependent manner for each applied voltage.

Figure 4 shows the time courses for the transport ofpentazocine from the donor compartment to the

17PENTAZOCINE TRANSPORT BY AC IONTOPHORESIS

Fig. 2. Diagram of a square-wave AC with an A% duty cycle at 1MHz. An A% duty cycle represents the ratio of the positive cycle tothe full cycle. The ratio of the positive cycle was adjusted between 50-55%.


Fig. 3. Relationship between time and pentazocine concentration at a 55% duty cycle under the application of 10 V, 25V and 40 V and under passive diffusion. Symbols in the graphs denote the measured values and the four lines are linearfits of the measured values using the least-squares method (LSM). The pentazocine concentration increased dependingon the time for each applied voltage.

Fig. 4. Relationship between time and pentazocine concentration under the application of 40V with duty cycles of 50%,51%, 52%, 53%, 54% or 55% and under passive diffusion. Symbols in the graphs denote the measured values and theseven lines are linear fits of the measured values using the least-squares method (LSM). The pentazocine concentrationincreased depending on the time for each duty cycle.

receptor compartment under the application of 40 Vwith duty cycles of 50%, 51%, 52%, 53%, 54% and55% at 1 MHz and under passive diffusion for 60 min-utes. The pentazocine concentration in the receptorcompartment increased in a time-dependent mannerfor each duty cycle.

3.2. Relationship between the duty cycle and pen-tazocine transportation

Figure 5 shows the relationships between the dutycycle and the mean pentazocine flux after the applica-tion of 10 V, 25 V or 40 V at 1 MHz for 60 minutes.Pentazocine fluxes showed a positive linear correlationwith the duty cycle under the application of 10 V (R2 =0.93, p = 0.0020), 25 V (R2 = 0.84, p = 0.0095) and 40V (R2 = 0.81, p = 0.0014). The maximum increase inthe pentazocine flux was obtained under the applicationof 40 V with a 55% duty cycle. The average penta-zocine flux from the donor compartment to the receptorcompartment under the application of 40 V with a 55%duty cycle for 60 minutes was 0.461 nmol/hr/cm2

(n=5). This value was nearly 3-fold the average penta-zocine flux under passive diffusion. The pentazocineflux was nearly 5-fold at 15 minutes, nearly 4-fold at 30minutes and 3.2-fold at 45 minutes under the applica-

tion of 40 V with a 55% duty cycle, compared with thepentazocine flux under passive diffusion. Under theapplication of 40 V, higher duty cycle accelerated thetransportation of pentazocine molecules to the receptorcompartment.

3.3. Relationship between the voltage and penta-zocine transportation

Figure 6 shows the relationships between the voltageand the pentazocine flux with duty cycles of 50%, 51%,52%, 53%, 54% or 55% at 1 MHz for 60 minutes.Pentazocine fluxes showed a strong linear correlationwith the voltage with duty cycles of 53% (R2 = 0.91, p =0.045), 54% (R2 = 0.97, p = 0.013) and 55% (R2 = 0.98,p = 0.012), although p-values were larger than 0.05with the voltage with duty cycles of 50% (R2 = 0.71, p =0.16), 51% (R2 = 0.88, p = 0.060) and 52% (R2 = 0.84,p = 0.086). The efficiency of pentazocine transportationtended to depend on the voltage although the depen-dence was not statistically significant with the smallduty cycles of 50%, 51% and 52%. The average fluxunder the application of 10 V with a 55% duty cycle for60 minutes was nearly 1.4-fold of that under passive dif-fusion. The average flux under 25 V with a 55% dutycycle was nearly 2-fold of that under passive diffusion


Fig. 5. Relationship between the duty cycles and the pentazocine flux under the application of 10 V, 25 V and 40 V.Symbols in the graphs denote the measured values and the three lines are linear fits of the measured values using theleast-squares method (LSM). For each of the applied voltages, the largest average pentazocine flux after 60 minutes wasobtained when a 55% duty cycle was applied.

for 60 minutes. The average flux under 40 V with a 55%duty cycle was nearly 3-fold of that under passive dif-fusion for 60 minutes.

3.4. Lactic acid concentration in the donor com-partment for 60 minutes

Figure 7 shows the correlation between the dutycycles and the lactic acid concentrations in the donorcompartment after iontophoresis for 60 minutes. Thelactic acid concentrations in the donor compartmentand the duty cycles had little linear correlation under theapplication of 10 V (R2 = 0.12, p = 0.51), 25 V (R2 =0.0013, p = 0.95) and 40 V (R2 = 0.15, p = 0.45).

3.5. pH of the receptor and donor compartments Table 1 shows the pH and the percentage of penta-

zocine ionization in the receptor and donor compart-ments after 60 minutes of iontophoresis. At pH 7.5, thepercentage of pentazocine ionization was greaterthan 95%. The pH in the receptor compartment underthe application 25 V with a 55% duty cycle and underthe application of 40 V with a 54% or 55% duty cyclewas larger than pH 7.5. In the donor compartment after60 minutes of iontophoresis, a pH of below 3.5 was


Fig. 6. Relationship between the voltage and the pentazocine flux with duty cycles of 50%, 51%, 52%, 53%, 54% or 55%.Symbols in the graphs denote the measured values and the six lines are linear fits of the measured values using the least-squares method (LSM). A voltage-dependence for the transportation efficiency of pentazocine is seen.

Table 1. Measured pH changes and calculated percentage of pen-tazocine ionization on the receptor and donor compartments after 60minutes of square-wave AC application.

observed only under the application 40 V with a 55%duty cycle.

Figure 8 shows the relationships between the dutycycle and pH after the application of 10 V, 25 V and 40V at 1 MHz for 60 minutes. The values of pH showedpositive linear correlations with the duty cycle under theapplication of 10 V (R2 = 0.70, p = 0.038), 25 V (R2 =0.87, p = 0.0069) and 40 V (R2 = 0.98, p = 0.00019).

Figure 9 shows the relationships between the voltageand pH with duty cycles of 50%, 51%, 52%, 53%, 54%or 55% at 1 MHz for 60 minutes. The values of pHshowed strong linear correlations with the voltage withduty cycles of 54% (R2 = 0.92, p = 0.042) and 55% (R2

= 0.98, p = 0.010), although p-values were larger than0.05 with duty cycles of 50% (R2 = 0.32, p = 0.43), 51%(R2 = 0.38, p = 0.38), 52% (R2 = 0.74, p = 0.14) and53% (R2 = 0.85, p = 0.078). The pH changes in thereceptor and donor compartments tended to increaseon the applied voltage with duty cycles of 54% and55%, although the correlations were not statistically sig-nificant with the small duty cycles of 50%, 51%, 52%and 53%. The maximum pH change was observedunder the application of 40 V with a 55% duty cycle.The percentage of pentazocine ionization in the

receptor compartment was 13.7% under the applicationof 40 V with a 55% duty cycle. Thus, pentazocine ion-ization in the receptor compartment was stronglyinfluenced by square-wave AC iontophoresis under theapplication of a high voltage with a high duty cycle.

3.6. AC waveform across the membraneFigure 10 shows a waveform through a cellophane

membrane under the application of 40 V with a 55%duty cycle at 1 MHz. The waveform was slightlydeformed. This deformation of the waveform had littleeffect on the duty cycle because of the waveform’s peri-odicity. For example, the waveform shown in Fig. 10gives a ratio

area in the positive side / total area for 1 period =55.6%

This value is close to the duty cycle (55%).

4. Discussion

4.1. Waveform, frequency and voltage In the present study, a square-wave AC with 6 kinds

of duty cycles adjusted at a frequency of 1 MHz was


Fig. 7. Relationship between the duty cycles and the lactic acid concentrations under the application of 10 V, 25 V and 40V. The lactic acid concentration was not dependent on the duty cycle.


Fig. 8. Relationship between the duty cycles and pH under the application of 10 V, 25 V and 40 V. Symbols in the graphsdenote the measured values and the three lines are linear fits of the measured values using the least-squares method(LSM).

Fig. 9. Relationship between the voltage and pH with duty cycles of 50%, 51%, 52%, 53%, 54% or 55%. Symbols in thegraphs denote the measured values and the six lines are linear fits of the measured values using the least-squares method(LSM).

applied for pentazocine iontophoresis. DC iontophore-sis causes a polarization of the skin and electrode sur-face that is oriented in the direction opposite to theapplied field. The skin acts as a capacitor in an electriccurrent, which leads a decrease of the effective currentwith increasing periods of continuous DC application.To avoid this polarization, the use of pulsed DC ion-tophoresis or AC iontophoresis has been studied.Minimal electrode polarization is produced duringhighfrequency AC iontophoresis, because the polarityperiodically alternates. However, the efficiency ofionic transportation via electrorepulsion is inferior to thatenabled by DC iontophoresis because of the periodicpolarity alternation. That is why we attempted to applya square-wave AC with an adjusted duty cycle for pen-tazocine transportation. The DC component of thesquare wave induces an electrorepulsive effect, eventhough the polarity alters periodically in square-waveACs with duty cycles. A square waveform is consideredto be a superposition of sine waves. A Fourier seriesexpansion of a square wave is made up the a sum ofsine waves. Square waves with adjusted duty cyclesare thus composed of a DC component and a sinewave component. The DC component was given as theconstant term of the Fourier series expansion of thesquare wave:

b0 = ( 2p－1 ) E (1)

where b0 is the DC component, p is one-hundredth ofthe duty cycle percentage (%), and E is the applied volt-age. According to equation (1), the DC component isproportional to the applied voltage and a linear functionof the duty cycle. Thus, when a higher voltage with ahigher duty cycle is applied, the DC component would

be higher. Pentazocine transportation was thusenhanced, depending on the time and the voltage,because the square-wave AC included a DC compo-nent. In addition, an increased duty cycle increases theDC component, resulting in the acceleration of penta-zocine transportation. Transport efficiency dependsmainly on polarity, charge and mobility of the chargedspecies, as well as the electrical duty cycle and thecomponents of the formulation23. So far, various wave-forms have been applied to avoid polarization of theelectrodes and skin in iontophoresis studies, includingan AC sawtooth waveform with DC10 or pulsed DC5,17.

The frequency and voltage of the electrical currentalso influence the efficiency of drug delivery. Someresearchers used high frequencies of 2 to 50 KHz andothers low frequencies of 1/125 Hz and 12.5 Hz13,17,18.

The electrolysis of water on the surface of electrodeincreases at elevated voltages. During clinical use,adverse effects like skin irritation, chemical burning, andredness occur because of the electrolysis of water atelevated voltages in pulsed DC or AC with pulsed DC.

In previous studies, we successfully transportedlidocaine ions using sine-wave AC iontophoresisunder various frequencies and voltages, both in vitroand in vivo24-26. Izumikawa reported that the mosteffective conditions for lidocaine transportationthrough a cellophane membrane were 25 V of electricvoltage at a frequency of 1 MHz. Pentazocine wasselected in the present study because it has a similarmolecular size and electrical charge to lidocaine. A dif-ferent waveform was applied in the present studybecause the efficiency of pentazocine transportationusing only AC iontophoresis was not as high as that inthe previous study using lidocaine. Square-wave AC


Fig. 10. Waveform through a cellophane membrane under the application of 40 V witha 55% duty cycle at 1 MHz.

iontophoresis with an adjusted duty cycle exhibited bet-ter performance for the pentazocine transportation.

4.2. Temperature of solution The diffusion coefficient was determined using the

Nernst-Einstein relationship, as follows:

D = kT－q

Ò (2)

where D is the diffusion coefficient, k is theBoltzman constant, T is the absolute temperature, Ò ismobility and q is the charge of the ion. Equation (2)shows that D is proportional to T. Thus, the diffusioncoefficient would be seriously affected by largechanges in temperature.

4.3. pH changesThe pH in the receptor compartment showed no sig-

nificant changes under the application of each voltagewith a low duty cycle, but was significantly elevatedunder the application of 40 V with duty cycles of 54% or55%. The latter findings suggest that the water in thecompartments was electrolyzed, resulting in the pro-duction of OH- in the receptor compartment. Thereaction rate of electrolysis increased depending on thevoltage and the duty cycle. The production of ions mayreduce the flux of similarly charged solute ions.Specifically, H+ ions compete with pentazocine ionsunder such circumstances. Additive competitive ionsreduce the iontophoretic drug flux because they carry afraction of the total current. The use of a buffer may berequired to avoid pH changes. The chemical propertiesof pentazocine are also related to the pH change. Sincepentazocine is insoluble in water, pentazocine exists inan aqueous solution as an ionic compound that disso-ciates into cations with the addition of lactic acid.Pentazocine is a weak base with a pKa value of 8.88.When the pH of an aqueous solution with a weaklybasic drug approaches the pKa, a very pronouncedchange in the ionization of the drug occurs. The pHchange in the solution influences the ionization of thedrug in a charged state. Cations are attracted to thecathode and repelled from the anode. The significantpH change influenced the dissociation of pentazocineand the adaptability of this method for clinical trials. Themaximum significant elevation in the pH wasobserved under the application of 40 V with a 55% dutycycle. Thus, practical voltages and duty cycles must beselected from within a range of clinically safe condi-tions. At clinical trials, the system with a safety devicethat controls pH elevation is required not to cause elec-trochemical burns.

In the case of square-wave AC iontophoresis with a50% duty cycle, no pH change in the solution wasobserved. The present study therefore suggests thatsquare-wave AC iontophoresis under conditions otherthan 40 V with duty cycles of 54% and 55% or 25 Vwith a duty cycle of 55% may be used in clinical trialswithout the need for an additive buffer with low mobilityor conductivity; however an additive buffer would berequired for pH control under the application of 40 Vwith a duty cycle of 54% or 55% or under the applica-tion of 25 V with a 55% duty cycle.

4.4. ElectrodesTwo types of electrodes can be used for iontophore-

sis: platinum (Pt) electrodes and silver/silver chloride(Ag/AgCl) electrodes. Pt electrodes were employed inthe present study because the Pt electrodes them-selves do not absorb or release any ions, preventingthe production of competitive ions except the conditionwith high voltage and high duty cycle. In our previousstudy, Pt electrodes were used both in vitro and in vivo.AC iontophoresis with symmetrically alternating polar-ity minimized the electric polarization of the Pt elec-trodes. The DC component, however, introduces theelectrolysis of water. Inert electrodes like Pt elec-trodes have a major disadvantage in that they inducethe electrolysis of water, resulting in the production ofH+ at the anode and OH- at the cathode because theredox-potential of the Pt electrodes is higher than thatof water. The production of these ions may reduce theflux, similar to the effect of charged solute ions,requiring the use of a buffer to avoid pH changes27. Inthe present study, the pH change depended onincreases in the voltage and duty cycles in the receptorcompartment. A buffer solution is commonly added toneutralize pH changes resulting from electrolysis inboth in vitro and in vivo iontophoresis studies; however,a buffer solution was not added in the present study toenable the pH changes to be observed. Ag/AgCl elec-trodes are commonly used because these electrodesare resistant to pH changes as a result of theirreversibility in DC iontophoresis; however, Ag/AgClelectrodes have two disadvantages: the absorption ofthe drug onto the electrodes and the release of chlo-rides at the cathode27. Ag and Ag/Cl electrodes are notcommonly used for AC iontophoresis. The use ofAg/AgCl electrodes was not necessary because thehydrogen and hydroxide ions that are generated atthese electrodes would not accumulate under theapplication of a symmetric bipolar AC field22. On theother hand, Ag/AgCl electrodes have been used for


alternating-pulse AC iontophoresis containing a DCcomponent. In this study, the solution in the receptorcell initially contained a very small number of ions, andthe electrode in the receptor cell acted as both a cath-ode and an anode because AC currents were used.The reaction at the Ag/AgCl electrode is as follows:

AgCl + e-→ Ag +Cl-

This is why Ag/AgCl electrodes are usually used inthe presence of an abundance of Cl- ions. In the pre-sent study, we utilized a Pt electrode, which is hardlyionized. If Ag and Ag/AgCl electrodes had been used,the pH change may have been moderated to somedegree by the abundance of Cl- ions. Platinum elec-trodes, on the other hand, release ions, (i.e., H+ andOH- ions) that may compete with pentazocine in thepresence of high voltages and high duty cycles. Toavoid this problem, a buffer solution with a lowermobility or conductivity than pentazocine is needed.Further investigation as to which type of electrodetransports pentazocine most efficiently is needed.

4.5. Efficiency of pentazocine transportationThe present study revealed that pentazocine trans-

ported through a cellophane membrane by square-wave AC iontophoresis with an adjusted duty cycle at afrequency of 1 MHz with dependence on the time, volt-age and duty cycle. Maximum pentazocine trans-portation was obtained under the application of 40 Vwith a 55% duty cycle; however, a remarkable elevationin the pH occurred under those conditions. Variouswaveforms such as pulsed DC, AC with DC offset andAC have been applied to avoid the polarization at skinand electrode, and to increase the duration of currentapplication. An appreciable increase in drug trans-portation, relative to continuous DC, has been reportedusing pulsed DC frequencies in the range of 1 to 40kHz and duty cycles ranging from 80% to 10%5,16,28,29,with current densities on the order of 0.16 - 0.33mA/cm2 5,28,29. Okabe et al. succeeded in delivering abeta-blocker into the human skin using a pulsed currentwith a 20% duty cycle30. Pikal et al. reported that apulsed current with an 80% duty cycle could enhanceglucose delivery to hairless mouse skin at a frequencyof 2 kHz and a current density of 0.1 mA/cm2 17.Ishikawa et al. succeeded in the transportation ofphthalic acid, benzoic acid, and verapamil through ratskin by using pulsed DC with a 50% duty cycle that wasperiodically reversed at a frequency of 4 kHz and a volt-age of 10 V. The cumulative amounts of permeatedmolecules and the permeability coefficients were

apparently high when switching intervals with shortperiods were used31.

The present study revealed that a high duty cycleenhanced the transport of pentazocine at high voltages.The results showed that the DC component of squarewaves applied with specific duty cycles and at specificvoltages contributed the enhancement of pentazocinetransportation, despite the periodic polarity changes.The transportation of pentazocine was affected byeven small changes in the duty cycle because of thevery high frequency of 1 MHz. Some reports on ion-tophoresis have described the use of AC or DC in com-bination with AC. Howard et al. succeeded in deliveringhydroxocobalamin (B12) using AC iontophoresis at a lowfrequency (1/120 Hz) for 2 and 4 hours13. Previously, wesuccessfully delivered lidocaine using AC alone at100 Hz - 1 MHz in vitro and at 1kHz in vivo24-26. A mod-est “off phase” time and polarity alternation in pulsedDC and low-frequency AC applications can preventpolarization, enabling longer periods of current appli-cation. Square wave or sawtooth wave AC with DC hasbeen applied for the delivery of mannitol, tap water, ortetraethyl ammonium (TEA), resulting in effectivetransportation of them10,18,21,22. The results of ion-tophoresis with a square-wave AC and an adjusted dutycycle were similar to those for square-wave AC withDC22, although a different frequency was used: fre-quencies of less than 50 kHz were used in most of theprevious studies, while a very high frequency of 1 MHzwas employed in the present study. As shown in theprevious studies, the DC component of the squarewave in combination with an AC plays a significant rolein drug transportation and in minimizing polarization.The effect on drug transportation is determined by var-ious factors, including waveform, voltage, frequency,ionized properties, pH of the medium, molecularweight and duration of the current application. The pre-sent study suggested that a voltage of 25 V with a 54%duty cycle or a voltage of 40 V with a 53% duty cyclewould be practical because of the slight pH changesthat occur under these conditions.

4.6. Effect of components other than pentazocineon pentazocine transportation

According to the results of the lactic acid experi-ments, iontophoresis influences the transportation oflactic acid. The effect of AC iontophoresis on thetransportation of lactic acid was, however, not asgreat as the effect on pentazocine transportation. InSebastiani’s study32 the flux of a cation drug (buspirone)was not enhanced by the presence of lactic acid.


Thus, pentazocine, which also is a cation drug, isunlikely to be influenced by the presence of lactic acid.The transport of Na+ and Cl-, both of which are highlymobile, is also expected to be promoted by AC ion-tophoresis, as described in Shibaji’s study33. Althoughthe influence of a DC component has not been studied,Na+ and Cl- transport was not analyzed in this study.The effect of these ions on the transportation of penta-zocine will need to be investigated in the future.

4.7. Possible transportation mechanismOnly a few studies on the mechanism of ion trans-

portation using AC iontophoresis have been made. Inthe present study, electrorepulsion and electroosmosisseemed thought to play essential roles in pentazocinetransportation, because of the square waves with a DCcomponent. Iontophoresis enhances drug deliveryacross membranes by three principal mechanisms:electrorepulsion, electroosmosis, and electroporation.However, electroporation plays a minimal role in drugdelivery across artificial membranes like cellophanemembranes. Electrorepulsion is the primary enhancingmechanism responsible for the transportation of ioniccompounds. In electrorepulsion, charged substancesare repelled from electrodes with the same polarity asthe charged substances and attracted to electrodeswith the opposite polarity34. According to electrorepul-sion, the positively charged pentazocine ions in thedonor compartment would be similarly repelled into andthrough the membrane during the positive phase of ACiontophoresis. Iontophoresis including a DC componentmay yield an impact energy like a pulsed DC ion-tophoresis28,29. Some researchers suggest that animpact energy concept does not apply to iontophore-sis17.

An electrically driven flow of ions across a membranewith a net charge can induce the coupled flow of sol-vent, called electroosomosis34. Electroosmosis pro-duces a bulk motion of the solvent itself that carriesions or neutral species, within the solvent stream35.Electroosmosis would play an important role only dur-ing the positive phase, like electrorepulsion21. When theconcentration of the ionized drug is very high, elec-troosomotic flow has a very small effect on drug flux,because the drug ions carry most of the current36. ACelectric fields provide little additional electroosmotictransport enhancement over that provided by the highDC offset21. Changes of pH at the electrodes can alterthe electroosmosis effect, because such changesaffect molecular ionization. The relative contributions ofelectrorepulsion and electroosmosis to the total ion-

tophoresis flux may be altered by the pH of the solu-tions and by pentazocine ionization.

The increase in the ion transportation velocity as aresult of AC iontophoresis would partially be caused byan increase in the translational vibration energy of theions supplied by the applied AC electric field33. Sincepolarity changes at high frequencies can give vibra-tional energy to ions, such events not only enhance thetransfer velocity of ions, but also increase the collisionrate between ions, leading to an increase in penta-zocine transportation through the cellophane mem-brane.

Another model for AC iontophoresis has been pro-posed by Mollee et al.37. However, the conditions andresults of their study did not correspond with those ofours.

Acknowledgements

This work was supported by a Grants-in-Aid forScientific Research No.14207088, from Ministry ofEducation, Culture, Sports, Science and Technology,Japan.

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pentazocine transport by square-wave ac iontophoresis with an

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