J Med Dent Sci 2008; 55: 15–27
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, Japan
2) 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
So far, pentazocine iontophoresis has never
been studied, although pentazocine is widely
used in pain management. The purpose of this
study was to determine whether pentazocine
transportation through a cellophane membrane
could be enhanced using square-wave alternating
current (AC) iontophoresis with an adjusted duty
cycle and dependence on the voltage and the duty
cycle. Voltages of 10, 25 and 40 V with duty cycles
of 50%, 51%, 52%, 53%, 54% and 55% were
applied for 60 minutes at a high frequency of 1 MHz
to diffusion cells on both sides of a cellophane
membrane. The donor compartment was filled
with a solution containing pentazocine. Squarewave AC iontophoresis with an adjusted duty
cycle enhanced pentazocine transportation at
higher voltages and duty cycles. These results
suggested that the direct current (DC) component
of the square-wave AC played an important role in
enhancing pentazocine transportation despite
changes in polarity at very high frequency of
1MHz. The higher voltages and duty cycles
induced a pH change. The practical electrical conditions that could be applied clinically were 25 V
Corresponding Author: Saori Ogami
Mailing address: Section of Anesthesiology and Clinical Physiology,
Department of Oral Restitution, Division of Oral Health Science,
Graduate School, Tokyo Medical and Dental University, 1-5-45
Yushima, Bunkyo-ku, Tokyo, 113-8549, Japan
Tel: +81-3-5803-5552 Fax: +81-3-5803-0206
E-mail address: saryanph@tmd.ac.jp
Received September 6; Accepted November 30, 2007
with a 54% duty cycle or 40 V with a 53% duty
cycle.
Key words:
Pentazocine, Iontophoresis, Squarewave, Voltage, Duty cycle
1. Introduction
Pentazocine is a non-narcotic morphine analogue
that is widely used for the management of patients with
postoperative pain or initial carcinogenic pain1,2. The
routes of pentazocine administration in clinical situations are oral or injection. Because of a high first-pass
effect, only 11-32% of an oral dose enters the systemic
circulation in patients3,4. Though the injection of pentazocine allows a rapid onset of pain relief, the needle
insertion itself is painful and stressful for the patients. A
transdermal route for pentazocine administration may
be a good alternative to avoid these problems.
Classical
transdermal
administration
without
enhancers would be problematic because of the low
permeability of the skin and the prolonged lag time
caused by the barrier function of the stratum
corneum.
Iontophoresis can be used to enhance transdermal
drug delivery. Local anesthetics, steroids, NSAIDS, opioids, peptides, and so on have been administered using
transdermal drug delivery5,6. The iontophoresis of
local anesthetics has been used practically in clinical
situations7-9. Iontophoresis using an AC and DC offset
16
S. OGAMI et al.
has also been used for the treatment of patients with
hyperhidrosis10. For example, an iontophoretic transdermal system of fentanyl hydrochloride, which is
patient-activated, has been used in humans11. The
symmetrical nature of iontophoresis, the nature that
ions are driven both into and out of the body, has also
been utilized to extract data from the body without blood
sampling. A reverse iontophoresis device has already
been introduced for glucose monitoring in patients
with diabetes12. Thus, iontophoretic transdermal delivery is very useful for improving the quality of life (QOL
) of patients. The iontophoresis of pentazocine has not
yet been studied (not even using DC iontophoresis).
Narcotics such as morphine and fentanyl are strictly
regulated during clinical use. Pentazocine, which is
non-narcotic analgesics, is easier to use than narcotic
analgesics.
Standard iontophoresis employs a continuous DC.
Continuous DC iontophoresis has some adverse
effects, including electrochemical burns, erythema,
and a reduced transportation effect as a result of the
13,14
polarization of the skin and the electrode . In an
effort to avoid these problems, AC and pulsed DC have
also been applied for iontophoretic drug delivery. AC
iontophoresis has the advantage of not causing electrochemical burns because the polarity of the current
alternates periodically. The optimal conditions for current application, including the waveform, the amplitude,
the voltage, and the frequency, have been investigated
for pulsed DC iontophoresis, DC iontophoresis with
alternating polarity, and AC iontophoresis with DC offset10,15-17,18-22. Though DC iontophoresis is more effective than AC iontophoresis for the transportation of
drugs, the duration of DC iontophoresis is limited to 10to-15-minute periods because of the electrochemical
burns produced by the hydrogen and hydroxide ions
that are generated by the DC current13. On the other
hand, AC iontophoresis enables a long duration of current application because of minimal polarization of the
skin and electrode and minimal skin irritation.
In this study, a square-wave with an adjusted duty
cycle of polarity alternation was employed to balance
the advantages of AC and DC iontophoresis for pentazocine transportation. We determined influence of
voltage and duty cycle on transport efficiency of pentazocine.
The focus of this research was to investigate
whether pentazocine can be transported efficiently
using square-wave AC iontophoresis with an adjusted
duty cycle and to find the optimal duty cycle and voltage
for pentazocine transportation.
J Med Dent Sci
2. Materials and Methods
2.1. Materials
Pentazocine (PENTAGINÑ injection) was purchased
from SANKYO Co., Ltd. (Tokyo, Japan). This commercial injection contains pentazocine (30 mg), lactic acid
(12 ÒL), and sodium chloride (2.8 mg) in a total volume
of 1 mL. The pentazocine injection was diluted with distilled water to adjust the pentazocine concentration to
0.5 mg/mL (pH 4.4).
2.2. Membrane
The cellophane membrane was about 36 Òm thick
with pore sizes of about 2-3 nm; these pore sizes were
one order of magnitude larger than the size of the ions
used in this study.
2.3. Permeation experiments
The cellophane membrane was placed between a
pair of acrylic diffusion cells with diameters of 15 mm
and length of each compartment was 10 mm.
Platinum plate electrodes (99.95% purity), with a
diameter of 15 mm and a thickness of 0.15 mm were
installed at opposite ends of the two compartments of
the diffusion cells, as seen in Fig. 1. The donor compartment was filled with 2 mL of solution containing
pentazocine (0.5 mg/mL), and the receptor compartment was filled with 2 mL of distilled water (Fig. 1). The
diffusion cells were set in a water bath, and the temperature in the receptor compartment was controlled so
as not to exceed 37°C.
A temperature probe (Model BAT-12; Physitemp,
USA) was inserted at the center of the receptor com-
Fig. 1. Diagram of the experimental system. The experimental system consisted of two diffusion cells, a temperature probe, a water
bath, a function generator and a high-speed amplifier.
PENTAZOCINE TRANSPORT BY AC IONTOPHORESIS
partment to monitor the temperature of fluid in it. A 20ÒL sample was taken from the center of the receptor
compartment every 15minutes during the application of
the electrical current. All experiments were replicated
five times. The solution in the receptor compartment
was not replaced because the amount of the sample,
20 ÒL, was negligibly small, compared with the volume
of 2 mL in the receptor compartment. The samples
were analyzed using a high performance liquid chromatography (HPLC) system. The pH in both compartments was measured using a pH meter (pHBOY-P2;
Shindengen Electric MFG. Co., Ltd, Japan) after sampling. The solutions in both compartments were not
stirred to avoid serious effects on diffusion.
The electric field was applied using a function generator (Model number 8116A; Hewlett Packard, Tokyo,
Japan) and a high speed power amplifier (Model number 4025; NF Electric Instruments, Kanagawa,
Japan). The waveform was monitored using an oscilloscope (Model number 54503A; Hewlett Packard,
Tokyo, Japan). A square-wave AC with duty cycles of
50%, 51%, 52%, 53%, 54% or 55% was applied for 60
minutes. The waveform from function generator with a
duty cycle of A% is shown in Fig. 2. The waveform
through the cellophane membrane was slightly
deformed at high frequencies because of the capacitance and inductance of the circuit.
Three different voltages, 10 V, 25 V and 40 V were
applied for each duty cycle condition. The frequency of
the applied electric field was kept at 1 MHz. The voltages and frequency were selected based on the
17
results of previous studies24-26.
2.4. Drug analysis
The concentrations of pentazocine was determined
using an HPLC system (Shimadzu, Japan) with an
appropriate column (Shim-packÑ VP-ODS, 150 mm×
4.6 mm, Shimadzu, Japan) and a mobile phase composed of 10mM phosphate buffer (pH 2.6): acetonitrile
(75:25) at a flow rate of 0.8 mL/min. The column was
maintained at a temperature of 40°C. UV detection was
performed at a wavelength of 278 nm. The flux
(nmol/hr/cm2) of pentazocine was calculated from the
cumulative amount of pentazocine transported to the
receptor compartment over a period of 60 minutes.
The lactic acid concentration in the donor compartment after 60 minutes of iontophoresis was determined
using an HPLC system with the same column as that
used for pentazocine and a mobile phase composed of
10 mM phosphate buffer (pH 2.6). UV detection was
performed at a wavelength of 210 nm.
2.5. Statistical analysis
All experiments but transportation of lactic acid
were replicated five times; the results were expressed
as the mean±standard error (S.E.). Statistical analyses
were performed by means of Microsoft Excel 2003.
Simple linear regression analysis was used to examine
the relationship between time and pentazocine concentration, the relationship between pentazocine fluxes
and predictor variables (duty cycle or applied voltage)
the relationship between lactic acid concentration and
duty cycle and the relationship between pH and predictor variables (duty cycle or applied voltage).
Coefficient of determinations (R2) were calculated
using the least-squares method. P-values to the slope
of <0.05 were regarded as statistically significant.
3. Results
Fig. 2. Diagram of a square-wave AC with an A% duty cycle at 1
MHz. An A% duty cycle represents the ratio of the positive cycle to
the full cycle. The ratio of the positive cycle was adjusted between 5055%.
3.1. The time courses for the transport of pentazocine
Figure 3 shows the time courses for the transport of
pentazocine from the donor compartment to the
receptor compartment under the application of 10 V, 25
V and 40 V with a 55% duty cycle at 1 MHz and under
passive diffusion for 60 minutes. The pentazocine
concentration in the receptor compartment increased in
a time-dependent manner for each applied voltage.
Figure 4 shows the time courses for the transport of
pentazocine from the donor compartment to the
18
S. OGAMI et al.
J Med Dent Sci
Fig. 3. Relationship between time and pentazocine concentration at a 55% duty cycle under the application of 10 V, 25
V and 40 V and under passive diffusion. Symbols in the graphs denote the measured values and the four lines are linear
fits of the measured values using the least-squares method (LSM). The pentazocine concentration increased depending
on 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 the
seven lines are linear fits of the measured values using the least-squares method (LSM). The pentazocine concentration
increased depending on the time for each duty cycle.
PENTAZOCINE TRANSPORT BY AC IONTOPHORESIS
receptor compartment under the application of 40 V
with duty cycles of 50%, 51%, 52%, 53%, 54% and
55% at 1 MHz and under passive diffusion for 60 minutes. The pentazocine concentration in the receptor
compartment increased in a time-dependent manner
for each duty cycle.
3.2. Relationship between the duty cycle and pentazocine transportation
Figure 5 shows the relationships between the duty
cycle and the mean pentazocine flux after the application of 10 V, 25 V or 40 V at 1 MHz for 60 minutes.
Pentazocine fluxes showed a positive linear correlation
with the duty cycle under the application of 10 V (R2 =
0.93, p = 0.0020), 25 V (R2 = 0.84, p = 0.0095) and 40
V (R2 = 0.81, p = 0.0014). The maximum increase in
the pentazocine flux was obtained under the application
of 40 V with a 55% duty cycle. The average pentazocine flux from the donor compartment to the receptor
compartment under the application of 40 V with a 55%
2
duty cycle for 60 minutes was 0.461 nmol/hr/cm
(n=5). This value was nearly 3-fold the average pentazocine flux under passive diffusion. The pentazocine
flux was nearly 5-fold at 15 minutes, nearly 4-fold at 30
minutes and 3.2-fold at 45 minutes under the applica-
19
tion of 40 V with a 55% duty cycle, compared with the
pentazocine flux under passive diffusion. Under the
application of 40 V, higher duty cycle accelerated the
transportation of pentazocine molecules to the receptor
compartment.
3.3. Relationship between the voltage and pentazocine transportation
Figure 6 shows the relationships between the voltage
and 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 correlation
with 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 0.05
with 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 transportation
tended to depend on the voltage although the dependence was not statistically significant with the small
duty cycles of 50%, 51% and 52%. The average flux
under the application of 10 V with a 55% duty cycle for
60 minutes was nearly 1.4-fold of that under passive diffusion. The average flux under 25 V with a 55% duty
cycle 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 the
least-squares method (LSM). For each of the applied voltages, the largest average pentazocine flux after 60 minutes was
obtained when a 55% duty cycle was applied.
20
S. OGAMI et al.
J Med Dent Sci
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 leastsquares method (LSM). A voltage-dependence for the transportation efficiency of pentazocine is seen.
for 60 minutes. The average flux under 40 V with a 55%
duty cycle was nearly 3-fold of that under passive diffusion for 60 minutes.
3.4. Lactic acid concentration in the donor compartment for 60 minutes
Figure 7 shows the correlation between the duty
cycles and the lactic acid concentrations in the donor
compartment after iontophoresis for 60 minutes. The
lactic acid concentrations in the donor compartment
and the duty cycles had little linear correlation under the
application 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 pentazocine ionization in the receptor and donor compartments after 60 minutes of iontophoresis. At pH 7.5, the
percentage of pentazocine ionization was greater
than 95%. The pH in the receptor compartment under
the application 25 V with a 55% duty cycle and under
the application of 40 V with a 54% or 55% duty cycle
was larger than pH 7.5. In the donor compartment after
60 minutes of iontophoresis, a pH of below 3.5 was
Table 1. Measured pH changes and calculated percentage of pentazocine ionization on the receptor and donor compartments after 60
minutes of square-wave AC application.
PENTAZOCINE TRANSPORT BY AC IONTOPHORESIS
21
Fig. 7. Relationship between the duty cycles and the lactic acid concentrations under the application of 10 V, 25 V and 40
V. The lactic acid concentration was not dependent on the duty cycle.
observed only under the application 40 V with a 55%
duty cycle.
Figure 8 shows the relationships between the duty
cycle and pH after the application of 10 V, 25 V and 40
V at 1 MHz for 60 minutes. The values of pH showed
positive linear correlations with the duty cycle under the
application 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 voltage
and pH with duty cycles of 50%, 51%, 52%, 53%, 54%
or 55% at 1 MHz for 60 minutes. The values of pH
showed strong linear correlations with the voltage with
duty cycles of 54% (R2 = 0.92, p = 0.042) and 55% (R2
= 0.98, p = 0.010), although p-values were larger than
0.05 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) and
53% (R2 = 0.85, p = 0.078). The pH changes in the
receptor and donor compartments tended to increase
on the applied voltage with duty cycles of 54% and
55%, although the correlations were not statistically significant with the small duty cycles of 50%, 51%, 52%
and 53%. The maximum pH change was observed
under the application of 40 V with a 55% duty cycle.
The percentage of pentazocine ionization in the
receptor compartment was 13.7% under the application
of 40 V with a 55% duty cycle. Thus, pentazocine ionization in the receptor compartment was strongly
influenced by square-wave AC iontophoresis under the
application of a high voltage with a high duty cycle.
3.6. AC waveform across the membrane
Figure 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 slightly
deformed. This deformation of the waveform had little
effect on the duty cycle because of the waveform’s periodicity. For example, the waveform shown in Fig. 10
gives 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
22
S. OGAMI et al.
J Med Dent Sci
Fig. 8. Relationship between the duty cycles and pH 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 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 the
graphs denote the measured values and the six lines are linear fits of the measured values using the least-squares method
(LSM).
PENTAZOCINE TRANSPORT BY AC IONTOPHORESIS
23
Fig. 10. Waveform through a cellophane membrane under the application of 40 V with
a 55% duty cycle at 1 MHz.
applied for pentazocine iontophoresis. DC iontophoresis causes a polarization of the skin and electrode surface that is oriented in the direction opposite to the
applied field. The skin acts as a capacitor in an electric
current, which leads a decrease of the effective current
with increasing periods of continuous DC application.
To avoid this polarization, the use of pulsed DC iontophoresis or AC iontophoresis has been studied.
Minimal electrode polarization is produced during
highfrequency AC iontophoresis, because the polarity
periodically alternates. However, the efficiency of
ionic transportation via electrorepulsion is inferior to that
enabled by DC iontophoresis because of the periodic
polarity alternation. That is why we attempted to apply
a square-wave AC with an adjusted duty cycle for pentazocine transportation. The DC component of the
square wave induces an electrorepulsive effect, even
though the polarity alters periodically in square-wave
ACs with duty cycles. A square waveform is considered
to be a superposition of sine waves. A Fourier series
expansion of a square wave is made up the a sum of
sine waves. Square waves with adjusted duty cycles
are thus composed of a DC component and a sine
wave component. The DC component was given as the
constant term of the Fourier series expansion of the
square wave:
b0 = ( 2p−1 ) E
(1)
where b0 is the DC component, p is one-hundredth of
the duty cycle percentage (%), and E is the applied voltage. According to equation (1), the DC component is
proportional to the applied voltage and a linear function
of the duty cycle. Thus, when a higher voltage with a
higher duty cycle is applied, the DC component would
be higher. Pentazocine transportation was thus
enhanced, depending on the time and the voltage,
because the square-wave AC included a DC component. In addition, an increased duty cycle increases the
DC component, resulting in the acceleration of pentazocine transportation. Transport efficiency depends
mainly on polarity, charge and mobility of the charged
species, as well as the electrical duty cycle and the
components of the formulation23. So far, various waveforms have been applied to avoid polarization of the
electrodes and skin in iontophoresis studies, including
an AC sawtooth waveform with DC10 or pulsed DC5,17.
The frequency and voltage of the electrical current
also influence the efficiency of drug delivery. Some
researchers used high frequencies of 2 to 50 KHz and
others low frequencies of 1/125 Hz and 12.5 Hz13,17,18.
The electrolysis of water on the surface of electrode
increases at elevated voltages. During clinical use,
adverse effects like skin irritation, chemical burning, and
redness occur because of the electrolysis of water at
elevated voltages in pulsed DC or AC with pulsed DC.
In previous studies, we successfully transported
lidocaine ions using sine-wave AC iontophoresis
under various frequencies and voltages, both in vitro
and in vivo24-26. Izumikawa reported that the most
effective conditions for lidocaine transportation
through a cellophane membrane were 25 V of electric
voltage at a frequency of 1 MHz. Pentazocine was
selected in the present study because it has a similar
molecular size and electrical charge to lidocaine. A different waveform was applied in the present study
because the efficiency of pentazocine transportation
using only AC iontophoresis was not as high as that in
the previous study using lidocaine. Square-wave AC
24
S. OGAMI et al.
iontophoresis with an adjusted duty cycle exhibited better performance for the pentazocine transportation.
4.2. Temperature of solution
The diffusion coefficient was determined using the
Nernst-Einstein relationship, as follows:
kT
D =−Ò
(2)
q
where D is the diffusion coefficient, k is the
Boltzman constant, T is the absolute temperature, Ò is
mobility and q is the charge of the ion. Equation (2)
shows that D is proportional to T. Thus, the diffusion
coefficient would be seriously affected by large
changes in temperature.
4.3. pH changes
The pH in the receptor compartment showed no significant changes under the application of each voltage
with a low duty cycle, but was significantly elevated
under the application of 40 V with duty cycles of 54% or
55%. The latter findings suggest that the water in the
compartments was electrolyzed, resulting in the production of OH in the receptor compartment. The
reaction rate of electrolysis increased depending on the
voltage and the duty cycle. The production of ions may
reduce the flux of similarly charged solute ions.
Specifically, H+ ions compete with pentazocine ions
under such circumstances. Additive competitive ions
reduce the iontophoretic drug flux because they carry a
fraction of the total current. The use of a buffer may be
required to avoid pH changes. The chemical properties
of pentazocine are also related to the pH change. Since
pentazocine is insoluble in water, pentazocine exists in
an aqueous solution as an ionic compound that dissociates 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 weakly
basic drug approaches the pKa, a very pronounced
change in the ionization of the drug occurs. The pH
change in the solution influences the ionization of the
drug in a charged state. Cations are attracted to the
cathode and repelled from the anode. The significant
pH change influenced the dissociation of pentazocine
and the adaptability of this method for clinical trials. The
maximum significant elevation in the pH was
observed under the application of 40 V with a 55% duty
cycle. Thus, practical voltages and duty cycles must be
selected from within a range of clinically safe conditions. At clinical trials, the system with a safety device
that controls pH elevation is required not to cause electrochemical burns.
J Med Dent Sci
In the case of square-wave AC iontophoresis with a
50% duty cycle, no pH change in the solution was
observed. The present study therefore suggests that
square-wave AC iontophoresis under conditions other
than 40 V with duty cycles of 54% and 55% or 25 V
with a duty cycle of 55% may be used in clinical trials
without the need for an additive buffer with low mobility
or conductivity; however an additive buffer would be
required for pH control under the application of 40 V
with a duty cycle of 54% or 55% or under the application of 25 V with a 55% duty cycle.
4.4. Electrodes
Two types of electrodes can be used for iontophoresis: platinum (Pt) electrodes and silver/silver chloride
(Ag/AgCl) electrodes. Pt electrodes were employed in
the present study because the Pt electrodes themselves do not absorb or release any ions, preventing
the production of competitive ions except the condition
with high voltage and high duty cycle. In our previous
study, Pt electrodes were used both in vitro and in vivo.
AC iontophoresis with symmetrically alternating polarity minimized the electric polarization of the Pt electrodes. The DC component, however, introduces the
electrolysis of water. Inert electrodes like Pt electrodes have a major disadvantage in that they induce
the electrolysis of water, resulting in the production of
H+ at the anode and OH- at the cathode because the
redox-potential of the Pt electrodes is higher than that
of water. The production of these ions may reduce the
flux, similar to the effect of charged solute ions,
requiring the use of a buffer to avoid pH changes27. In
the present study, the pH change depended on
increases in the voltage and duty cycles in the receptor
compartment. A buffer solution is commonly added to
neutralize pH changes resulting from electrolysis in
both in vitro and in vivo iontophoresis studies; however,
a buffer solution was not added in the present study to
enable the pH changes to be observed. Ag/AgCl electrodes are commonly used because these electrodes
are resistant to pH changes as a result of their
reversibility in DC iontophoresis; however, Ag/AgCl
electrodes have two disadvantages: the absorption of
the drug onto the electrodes and the release of chlorides at the cathode27. Ag and Ag/Cl electrodes are not
commonly used for AC iontophoresis. The use of
Ag/AgCl electrodes was not necessary because the
hydrogen and hydroxide ions that are generated at
these electrodes would not accumulate under the
application of a symmetric bipolar AC field22. On the
other hand, Ag/AgCl electrodes have been used for
PENTAZOCINE TRANSPORT BY AC IONTOPHORESIS
alternating-pulse AC iontophoresis containing a DC
component. In this study, the solution in the receptor
cell initially contained a very small number of ions, and
the electrode in the receptor cell acted as both a cathode and an anode because AC currents were used.
The reaction at the Ag/AgCl electrode is as follows:
AgCl + e- → Ag +ClThis is why Ag/AgCl electrodes are usually used in
the presence of an abundance of Cl- ions. In the present study, we utilized a Pt electrode, which is hardly
ionized. If Ag and Ag/AgCl electrodes had been used,
the pH change may have been moderated to some
degree by the abundance of Cl- ions. Platinum electrodes, on the other hand, release ions, (i.e., H+ and
OH- ions) that may compete with pentazocine in the
presence of high voltages and high duty cycles. To
avoid this problem, a buffer solution with a lower
mobility or conductivity than pentazocine is needed.
Further investigation as to which type of electrode
transports pentazocine most efficiently is needed.
4.5. Efficiency of pentazocine transportation
The present study revealed that pentazocine transported through a cellophane membrane by squarewave AC iontophoresis with an adjusted duty cycle at a
frequency of 1 MHz with dependence on the time, voltage and duty cycle. Maximum pentazocine transportation was obtained under the application of 40 V
with a 55% duty cycle; however, a remarkable elevation
in the pH occurred under those conditions. Various
waveforms such as pulsed DC, AC with DC offset and
AC have been applied to avoid the polarization at skin
and electrode, and to increase the duration of current
application. An appreciable increase in drug transportation, relative to continuous DC, has been reported
using pulsed DC frequencies in the range of 1 to 40
kHz and duty cycles ranging from 80% to 10%5,16,28,29,
with current densities on the order of 0.16 - 0.33
mA/cm2 5,28,29. Okabe et al. succeeded in delivering a
beta-blocker into the human skin using a pulsed current
with a 20% duty cycle30. Pikal et al. reported that a
pulsed current with an 80% duty cycle could enhance
glucose delivery to hairless mouse skin at a frequency
of 2 kHz and a current density of 0.1 mA/cm2 17.
Ishikawa et al. succeeded in the transportation of
phthalic acid, benzoic acid, and verapamil through rat
skin by using pulsed DC with a 50% duty cycle that was
periodically reversed at a frequency of 4 kHz and a voltage of 10 V. The cumulative amounts of permeated
molecules and the permeability coefficients were
25
apparently high when switching intervals with short
periods were used31.
The present study revealed that a high duty cycle
enhanced the transport of pentazocine at high voltages.
The results showed that the DC component of square
waves applied with specific duty cycles and at specific
voltages contributed the enhancement of pentazocine
transportation, despite the periodic polarity changes.
The transportation of pentazocine was affected by
even small changes in the duty cycle because of the
very high frequency of 1 MHz. Some reports on iontophoresis have described the use of AC or DC in combination with AC. Howard et al. succeeded in delivering
hydroxocobalamin (B12) using AC iontophoresis at a low
frequency (1/120 Hz) for 2 and 4 hours13. Previously, we
successfully delivered lidocaine using AC alone at
100 Hz - 1 MHz in vitro and at 1kHz in vivo24-26. A modest “off phase” time and polarity alternation in pulsed
DC and low-frequency AC applications can prevent
polarization, enabling longer periods of current application. Square wave or sawtooth wave AC with DC has
been applied for the delivery of mannitol, tap water, or
tetraethyl ammonium (TEA), resulting in effective
transportation of them10,18,21,22. The results of iontophoresis with a square-wave AC and an adjusted duty
cycle were similar to those for square-wave AC with
DC22, although a different frequency was used: frequencies of less than 50 kHz were used in most of the
previous studies, while a very high frequency of 1 MHz
was employed in the present study. As shown in the
previous studies, the DC component of the square
wave in combination with an AC plays a significant role
in drug transportation and in minimizing polarization.
The effect on drug transportation is determined by various factors, including waveform, voltage, frequency,
ionized properties, pH of the medium, molecular
weight and duration of the current application. The present study suggested that a voltage of 25 V with a 54%
duty cycle or a voltage of 40 V with a 53% duty cycle
would be practical because of the slight pH changes
that occur under these conditions.
4.6. Effect of components other than pentazocine
on pentazocine transportation
According to the results of the lactic acid experiments, iontophoresis influences the transportation of
lactic acid. The effect of AC iontophoresis on the
transportation of lactic acid was, however, not as
great as the effect on pentazocine transportation. In
Sebastiani’s study32 the flux of a cation drug (buspirone)
was not enhanced by the presence of lactic acid.
26
S. OGAMI et al.
Thus, pentazocine, which also is a cation drug, is
unlikely to be influenced by the presence of lactic acid.
The transport of Na+ and Cl-, both of which are highly
mobile, is also expected to be promoted by AC ion33
tophoresis, as described in Shibaji’s study . Although
the 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 pentazocine will need to be investigated in the future.
4.7. Possible transportation mechanism
Only a few studies on the mechanism of ion transportation using AC iontophoresis have been made. In
the present study, electrorepulsion and electroosmosis
seemed thought to play essential roles in pentazocine
transportation, because of the square waves with a DC
component. Iontophoresis enhances drug delivery
across membranes by three principal mechanisms:
electrorepulsion, electroosmosis, and electroporation.
However, electroporation plays a minimal role in drug
delivery across artificial membranes like cellophane
membranes. Electrorepulsion is the primary enhancing
mechanism responsible for the transportation of ionic
compounds. In electrorepulsion, charged substances
are repelled from electrodes with the same polarity as
the charged substances and attracted to electrodes
34
with the opposite polarity . According to electrorepulsion, the positively charged pentazocine ions in the
donor compartment would be similarly repelled into and
through the membrane during the positive phase of AC
iontophoresis. Iontophoresis including a DC component
may yield an impact energy like a pulsed DC iontophoresis28,29. Some researchers suggest that an
impact energy concept does not apply to iontophoresis17.
An electrically driven flow of ions across a membrane
with a net charge can induce the coupled flow of solvent, called electroosomosis34. Electroosmosis produces a bulk motion of the solvent itself that carries
ions or neutral species, within the solvent stream35.
Electroosmosis would play an important role only during the positive phase, like electrorepulsion21. When the
concentration of the ionized drug is very high, electroosomotic flow has a very small effect on drug flux,
because the drug ions carry most of the current36. AC
electric fields provide little additional electroosmotic
transport enhancement over that provided by the high
DC offset21. Changes of pH at the electrodes can alter
the electroosmosis effect, because such changes
affect molecular ionization. The relative contributions of
electrorepulsion and electroosmosis to the total ion-
J Med Dent Sci
tophoresis flux may be altered by the pH of the solutions and by pentazocine ionization.
The increase in the ion transportation velocity as a
result of AC iontophoresis would partially be caused by
an increase in the translational vibration energy of the
ions supplied by the applied AC electric field33. Since
polarity changes at high frequencies can give vibrational energy to ions, such events not only enhance the
transfer velocity of ions, but also increase the collision
rate between ions, leading to an increase in pentazocine transportation through the cellophane membrane.
Another model for AC iontophoresis has been proposed by Mollee et al.37. However, the conditions and
results of their study did not correspond with those of
ours.
Acknowledgements
This work was supported by a Grants-in-Aid for
Scientific Research No.14207088, from Ministry of
Education, Culture, Sports, Science and Technology,
Japan.
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