Model systems in iontophoresis — transport efficacy

Advanced Drug Delivery Reviews, 9 (1992) 265-287

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© 1992 Elsevier Science Publishers B.V. All rights reserved. / 0169-409X/92/$15.00 A D R 00124

Model systems in iontophoresis - transport efficacy B u r t o n H. Sage Jr. a a n d J i m E. R i v i e r e b aBecton Dickinson Research Center, Division of Becton Dickinson and Company, Research Triangle Park, NC, USA and bCutaneous Pharmacology & Toxicology Center, North Carolina State University, Raleigh, NC, USA (Received December 6, 1991) (Accepted January 7, 1992)

Key words: Controlled release; Electrochemical reaction; In vitro transport; In vivo transport; Iontophoresis; Ion transport; Novel drug delivery; Patient-controlled analgesia; Transdermal

Contents Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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II. Basis o f evaluating model systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. H o w much drug can be delivered by iontophoresis? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Example is Dilaudid T M iontophoresis attractive for non-invasive patient-controlled analgesia? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Key variables in the process o f iontophoresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

268 269

III. Model systems for studying efficiency o f iontophoresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. History o f model systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Basic in vitro experimental system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. In vitro experimental system with active electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. The Franz configuration o f the in vitro experimental system . . . . . . . . . . . . . . . . . . . . . . . . . 5. In vitro perfused systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. In vivo results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

273 273 274 275 278 279 283

IV. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

285

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abbreviations: IND/NDA, Investigational New Drug/New Drug Application; i.v., intravenous; PCA, patient-controlled anesthesia. Correspondence: B.H. Sage Jr., Becton Dickinson Research Center, Division of Becton Dickinson and Company, 21 Davis Drive, P.O. Box 12016, Research Triangle Park, NC 27709, USA. Fax: (1) (919) 5497572.

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Summary Experimental model systems used to study the transport efficacy of ionic compounds through skin during iontophoresis are reviewed. Emphasis has been placed on the ability of these model systems to predict transport efficacy in vivo. Comparative studies using these model systems show that systems based on excised skin can significantly underestimate drug flux achievable in vivo under identical formulation and electrical conditions and that the isolated perfused porcine skin flap model accurately predicts in vivo drug flux. Using the hypothetical example of the drug Dilaudid T M in an iontophoretic device for the non-invasive management of pain, the differences in transport efficacy determined with an excised skin model and the skin flap model are shown to significantly impact the estimation of the commercial viability of the device.

I. Introduction Whenever an electric field is established across a conducting medium, a force is exerted on the charged particles that exist in that medium. If the particles are free to move, the force results in an electric current. In metals, for example, in a copper wire, the force is exerted on conduction band electrons and the result is electronic current. In ionic solutions, such as physiologic saline, the force is exerted on ions and the result is ionic current. Iontophoresis is a hybrid of these two currents established by an electric field, as shown in Fig. 1. In this figure, a power source, e.g., a battery, is used to establish the electric field. The power source anode is at the highest potential and each successive point in this circuit, in the counter-clockwise direction, is at a lower potential. The cathode of the power source is the lowest potential point. The resulting electric fields thus cause electrons to migrate in the direction of

Anode e_~

Cathode Power Source

]

" Cathode

Reservoir Anode Reservoir Anode ~

Cathode ~ ~C÷

~C~ ,C

-"' ,"J

"' illlli? :ii:illllli:ii:illlllillllllll .................. C~-~ Fig. 1. Charged particle flow in an iontophoretic circuit.

M O D E L SYSTEMS IN IONTOPHORESIS

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the battery anode in the electronic portions of the circuit and causes ions to flow in the ionic solution portion of the circuit - positive ions moving toward the cathode and negative ions toward the anode. The ionic solution portion of the circuit, in a counter-clockwise direction, is comprised of the anode reservoir, the skin under the anode reservoir, hydrated tissue between the anode and cathode reservoirs, the skin under cathode reservoir, and the cathode reservoir. The current flowing in the ionic parts of the circuit has two components: cations (C +) moving toward the cathode and anions ( A - ) moving toward the anode. If there are more than one species of cation or anion, such as in the hydrated tissue, each species will contribute to the overall current depending on its concentration and mobility. At any surface drawn perpendicular to the direction of current flow, the total number of ions passing through that surface per unit time (in either direction) must equal the current being delivered by the battery. If the solution of ions in the anode reservoir contains a therapeutic ion which is a cation, then it will migrate in the direction of the cathode and hence into the skin. Similarly, if the cathode contains a therapeutic ion which is an anion, then it will migrate toward the anode and into the skin. Thus for ionic drugs, the applied electric field accomplishes drug delivery. Because of the ease with which the magnitude of the electric field can be changed, there are many advantages to this form of drug delivery [1]. Drug delivery by iontophoresis is not new. The basic mechanisms were clearly appreciated by Leduc [2] in his classic demonstrations during the turn of the century. Since Leduc's studies, additional early studies of therapeutic uses of this process were reported [3,4]. In spite of this work, iontophoresis has remained at the fringes of the medical arts. While the reasons for this underutilization are mostly speculative at this time, a review of the literature since the turn of the century suggests the following: - M o s t studies were in man and were for the most part uncontrolled. Administered dose and resulting serum concentrations were not determined. Therapeutic effect was subjectively evaluated frequently without placebo control. Adverse effects, if present, were either ignored or played down. - Model systems for developing an understanding of the process in terms of significant variables and theoretical limitations have not been available until the most recent decades. As man is notoriously variable in his absorption, distribution and elimination of therapeutic compounds and in the magnitude and variety of adverse effects, it is little wonder that an understanding of the process has awaited these model systems. During the past decade peer reviewed studies using model systems aimed at an understanding of the iontophoretic process began to emerge. As of this writing, reports of studies in man demonstrating the safety and efficacy, including bioavailability, of therapeutic compounds do not exist for iontophoresis. In order to bring iontophoresis into the mainstream of therapeutic thinking, -

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test systems and dosage forms must be developed which can stand the scrutiny of the I N D / N D A review process. The development of this quality of dosage form will require the use of model systems which permit not only the understanding of the fundamentals of the iontophoresis process, but an optimization of the operational variables to insure not only therapeutically useful and reproducible dosing, but efficient use of the electrical power source and the therapeutic compound. It is the goal of this chapter to review the model systems which have emerged in recent years, to present representative data from these model systems and to discuss the insights gained from these models in terms of the therapeutic efficacy of the process. It is recognized that these same model systems are also appropriate for studies aimed at elucidating the details of the iontophoretic process at the cellular and subcellular level, not only in terms of the route the ions take in their journey into and through the skin, but in terms of the variety of effects the ions may induce along the way. A review of the utility of model systems to gain this level of understanding is outside the scope of this review.

II. Basis of evaluating model systems The point of view adopted here is that to be useful a model system should allow prediction of the therapeutic utility of a dosage form in man. This therapeutic utility can be expressed in terms of achievable rate and reproducibility of drug administration and utilization of the power and therapeutic resource available in the dosage form. The rate and reproducibility are important because before a dosage form can be developed for a given therapeutic indication, not only the total dose, but the time course of administration of the total dose and its variability should be known. The importance of the time course of administration has emerged with the advent of passive transdermal systems and other zeroth-order delivery systems in terms of the potential for tolerance and the chronopharmacological benefit available from time-regulated delivery. The efficiency of utilization of the power and therapeutic resource are important because these efficiencies determine the overall size of the dosage form, the frequency of battery replacement and the overall system cost when a precious therapeutic compound is used. First and foremost then is the development of an experimental system which predicts the time course of drug administration in man. The other key parameters, such as overall bioavailability and dosage form size, are all calculations which can be made once the flux curve is known. Such a model system affords its user two luxuries - first, it generates a high level of confidence when the I N D / N D A path to product approval is contemplated and second, as data are generated over a diversity of therapeutic molecules, the feasibility of a product using a given molecule in a specific therapeutic indication can be estimated with a minimum of experimentation.

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II.1. How much drug can be delivered by iontophoresis? Since there is no point in developing a model system which predicts in vivo serum concentrations in man if these serum concentrations are subtherapeutic under the most optimistic conditions, the first question to be answered is: what serum concentrations can be expected from iontophoresis? In the following paragraphs, this question will be answered in terms of a 'steady-state' formalism. The reader is reminded that this formalism is only applicable to steady-state conditions. For many indications - indeed, the example used here the treatment must be expanded to include absorption and elimination kinetics to determine the iontophoresis system operational parameters required for rapid onset of the therapeutic action of the drug. To develop the steady-state formalism, we rely on Faraday's law. In its simplest form, Faraday's law relates the moles of ions that pass through any given plane in an ionic circuit to the electric current from the power source, the time the current flows and the charge per ion. Symbolically, this is written:

(Eqn.1)

Mj°c t Zij

where Mj is the number of moles of the jth ion, t is time, Z the valence (charge per ion) and ij the current carried by j t h species. Faraday experimentally determined the proportionality constant, now appropriately called Faraday's constant (59, so the relationship becomes:

MJ-ij St

(Eqn.2)

When more than one ion is moving in the ionic path, the total number of moles moving is written:

M = E MJ =

t ~-., ij

z-~ zJ

(Eqn.3)

As in the case in iontophoresis, one is often more interested in one of the species of ions than the others. If we let the plane in our ionic circuit be the skin under the active electrode (see Fig. 1), and let the subscript D refer to the drug species, then we can write: t iD MD -- 7-- Z---D

(Eqn.4)

In pharmacology, the amount of drug is usually specified in terms of units of mass (gram or milligram) instead of moles. Since mass (m) is equal to the compound formula weight x moles, Eqn. 4 can be written:

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mD

M W t iD

-- - _q-

(Eqn.5)

Z o

The units conventionally used are: M W for molecular weight (g/mol), t for time (s), Z for valence (unitless), y f o r Faraday's constant (coulombs/mol) and mD for mass (g). Eqn. 5 would allow us to calculate the mass of drug deliverable by iontophoresis if iD were known. Unfortunately, at this point in time, the only known way to determine io is by experiment (the appropriate model system for this determination will be described in a later section). But it is possible to express iD in terms of iontophoresis system parameters and hence express the dosage of the drug (mD) in terms of these same system parameters. Returning again to Fig. 1 and the plane of the skin under the active electrode, since the circuit is a simple series circuit, the current flowing out of the power source must add up to all of the individual ionic currents. That is: I = Z ij J

(Eqn.6)

where I is the electronic current. We can now define a current efficiency for drug transport, ED, in terms of the total iontophoretic current I as: ED ------iD= current efficiency I

(Eqn.7)

where iD = current carried by drug ions, one of the ij. Placing Eqn. 7 into Eqn. 5 we get: mD

=

MWt ZDY

E D I - -

(Eqn.8)

The final step is to write the total iontophoresis current as the product of electrode area A (cm 2) and the current density ID (mA/cm2), since experience has shown that skin irritation is related to current density ID and electrode area A relates to the total size of the device and customer acceptance. Thus: MWt mD = EDIDA ZDSV

(Eqn.9)

iD = E D I D A

(Eqn.10)

Note that from Eqn. 5:

As can be seen in Eqn. 10, iD is the product of the three major independent system variables: the current efficiency, the electrode area and the current

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density. As can be seen in the example below, iD can also be calculated from the therapeutic requirements and physicochemical properties of the molecule. Once iD is known, electrode area and current density can be parameterized in terms of current efficiency. H.2. Example - is Dilaudid T M iontophoresis attractive for non-invasive patientcontrolled analgesia? The first step to answer this question is to use Eqn. 5 to calculate the current needed to carry just the drug (iD). Solving for iD, we get: iD - -

mD F

t

ZD

MW

(Eqn.ll)

From the Merck manual we find that M W = 286 (Only the hydromorphone molecule enters the skin. The chloride counterion, being of opposite charge, is not transported into the skin. Hence it is inappropriate to use the formula weight for the salt.) and that ZD = + 1 (with a positive charge, delivery will be from the anode). 5r, Faraday's constant, is 96,500 coulombs/mol. The mass per time is the dosage required and comes from experience [6] using Dilaudid TM in subcutaneous infusion for patient-controlled analgesia. The value elected for this analysis is 5 rag/h*. Putting these values into Eqn. 11 as written above leads to an iD of about 500 gA. From Eqn. 10, it follows that: EDIDA = 500 gA

(Eqn. 12)

Eqn. 12 can be graphed parametrically as shown in Fig. 2. In Fig. 2, the horizontal axis is the current density in laA/cm 2 and the vertical axis is anode area in cm 2 (The total system area will be roughly 2.5 times the anode area in cm 2, since an indifferent electrode of roughly equal area must be provided and means for insulating the two electrodes and adhering the total system to the skin must be provided). Shown as parametric curves is Eqn. 12 for selected values of current efficiency ED. Note the lines in Fig. 2 are the locus of points where io is 500 p.A. Curves for other values of ED can also be plotted. The parametric curves in Fig. 2 are used in the following manner. Suppose that experimental data have shown that skin irritation occurs at current densities above 200 p.A/cm 2 and that the current efficiency is 10%. This leads directly to an anode area of 25 cm 2 or a total system area of about 60 cm 2. Alternatively, if market research shows that a device equal to or smaller than a ~The value of 5 mg/h for Dilaudid is high and amounts to 120 mg/day. The reasons for using this high a dosing rate are: (1) patient coverage the iontophoresis system should be able to handle virtually all patients and the range of patient requirements is large; and (2) bolus requirement in PCA, when the patient feels pain, the more rapid the analgesia, the better. Hence the need for a transient high iontophoresis flux.

272

B.H. S A G E Jr. A N D J.E. R I V I E R E 75 70 65 60 55 50 45 40 35 30 25 20 15 10

rent efficiency

5 100

200

300

400

500

600

700

800

1000

tolerable current density (laA/cm2) Fig. 2. Importance of current efficiency. Using the example of Dilaudid T M for non-invasive patientcontrolled analgesia, the parametric Eqn. 10 it) = E t ) l o A = 500 ~tA is graphically solved.

2 x 2 gauge pad (25 cm 2) would be desirable (anode area of 10 cm z) and the current density could not exceed 200 I~A/cm2, then Fig. 2 rapidly shows a need for a current efficiency of about 25%.

II.3. Key variables in the process of iontophoresis Eqn. 12 has yet another use beyond a parametric evaluation of the feasibility of a product concept. It points the way to fruitful areas of iontophoresis research. From Eqn. 12 and a steady-state analysis for any other therapeutic indication, the result is that the product of current efficiency, current density and anode area will always be equal to a constant. In general, smaller devices will be preferred by users to larger devices. Therefore, research into methods to increase the current efficiency and methods to increase the safe current density will result in smaller devices and/or broader utility of the technology. Finally, note that of the two areas of research, improvement of current efficiency provides the added bonus of lowering the total current requirement (as well as reducing the size of the device) which lengthens the battery life. These considerations led to research into current efficiency - how can current efficiency be measured?, what levels of current efficiency can be achieved in vivo? and what in vitro model system can be constructed that predicts the current efficiency determined in vivo?. The following paragraphs describe our experience in model system development.

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III. Model systems for studying efficiency of iontophoresis III.1. History of model systems As mentioned at the outset, the purpose of this chapter is a discussion of model systems with the goal of finding an in vitro system capable of predicting in vivo results. Traditionally, the appropriate model system for predicting delivery in man has been an in vivo animal model. This has been especially true for oral dosage forms, where studies in animals have justified studies in man. Unlike oral systems, however, the size of the administered dose for an iontophoretic system is not an independent variable. It is dependent on factors which include current, time, molecular weight and valence as formulated in the previous section. And it is dependent upon these factors through an experimentally determined factor, the current efficiency, which as yet cannot be calculated from established theory. Because the administered dose is not independently known, traditional dose-response studies cannot be performed. Since traditional dose-response studies cannot be performed, studies of iontophoresis have been limited to demonstrations of biological effects. These studies, which are numerous, are listed in several of the review articles on iontophoresis [1,7,8]. Examples of such biological effects range from death [2] to topical anesthesia [9] to antibiotic treatment of burns [10]. Efforts to measure the administered dose during these experiments have been frustrated by several factors, including: (1) episodes have been short, leading to very low administered doses; (2) the topical delivery leads directly to absorption by the systemic circulation, and enormous dilution - usually to concentrations in the blood below the limit of assay sensitivity; and (3) the concentration of the active ingredient in the device is sufficiently high and the amount of material delivered sufficiently small, that the delivered dose cannot even be measured by determining the amount of mass lost from the device. For these reasons, reports of administered dose are few and far between in the iontophoretic literature. Thus throughout the long history of iontophoresis there has been a need for a model system which enabled the investigator to quantify the amount of agent transported during an experiment. Only after a model system which permitted quantification of transported compound was identified could one proceed to refining such a system to one which permits the prediction of in vivo administration. One of the important steps in the direction of finding a model to quantitate transport rates was the development of the system shown in Fig. 3. In this system, a piece of skin is placed between a reservoir containing a drug formulation and a second reservoir without the drug. During an iontophoresis experiment, the second reservoir can be sampled and assayed to determine the rate of accumulation of the drug in that reservoir. This overcomes problems cited above. Long iontophoresis episodes can be conducted and there is no dilution into a large volume vasculature as there is in animals or humans. Dual reservoir systems to study electroosmosis and related electrokinetic phenomena in various porous membranes are well known [11]. The application

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Fig. 3. Basic in vitro excised skin experimental system.

of these dual reservoir experimental systems to the problem of ionic transport across skin began in the early 1980's [5]. Since then numerous reports have appeared [1,7,8] using these in vitro excised skin model systems. Since the major goal of this chapter is to trace the development of model systems in iontophoresis to the description of one which has now predicted in vivo results for several therapeutic molecules, attention will now be turned to the components of these in vitro systems and problems which arise when these components are improperly chosen. 111.2. Basic in vitro experimental system The prototypical in vitro excised skin model is shown in Fig. 3. It is characterized by seven key components: - a source of electric power. This power source provides sufficient voltage to cause the ionic current in the hydrated compartments to flow. - electrodes. These are the elements which transform electronic current to ionic current. There are at least: an anode and a cathode. magnetic stirrer table - to provide mixing of the ionic species in the ionic solutions. excised skin - this is the membrane whose ionic transport properties are being studied. It can be from any of several different species and is usually a few hundred ~tm thick. donor reservoir - usually an aqueous solution of the ionic species of interest. The transport of this species across the excised skin is typically the objective of the experiment. receptor reservoir - usually an isotonic solution of sodium chloride, since it represents the body. not shown - a water jacket to keep the solutions of ionic species at a constant temperature - usually about 35°C. In a typical experiment the glassware is assembled sandwiching the excised skin. The drug formulation is placed in the donor reservoir and normal saline is placed in the receptor reservoir. The platinum electrodes are connected to a -

-

-

-

-

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7 6

1

2

3

4

time (h) Fig. 4. Lidocainefluxusingthe basicin vitro excisedskinexperimentalsystem.The total currentwas 156 p.A,the area of 300 lamthickporcineskinwas 0.78 cm2, the 600 ~tldonor solutionwas 0.427 M lidocaine HCI and the receiversolutionwas 0.15 M NaCI. The experimentwas run for 150 min. power supply, which may be either constant current or constant voltage and after a short equilibration period to allow the setup to reach a stable temperature, the experiment is begun. Periodically, solution is withdrawn from the receptor reservoir (and replaced with additional normal saline) and assayed for the drug. In this manner, the accumulation of the drug in the receptor reservoir as a function of time can be documented. In our laboratory, this experimental model system, using lidocaine hydrochloride as the drug and a constant current power source, gNes the lidocaine flux as a function of time shown in Fig. 4. While it is very clear that lidocaine is transported to the receptor compartment, when the details of the process, in terms of the constancy of delivery and the efficiency of power utilization are considered, the process is quite impractical. It could never be used to deliver Dilaudid T M , as discussed in the first section, since the delivery rate is not constant and the maximum current efficiency of 5% implies a patch that is too large. It is results like this, primarily reports of low transport efficiency [12-14], which may be leading many to underestimate the commercial utility of iontophoresis. But as we now understand, these results (Fig. 4) underestimate the results which can be achieved when models more appropriate for the process are used. The remainder of this section will be devoted to describing the evolution of in vitro systems in our laboratory to one which has had success in predicting in vivo results for several compounds.

111.3. In vitro experimental system with active electrodes In Fig. 3, the donor compartment is shown to have the anode or positive electrode immersed in the drug solution. This is the requirement when a

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B.H. S A G E Jr. A N D J.E. R I V I E R E

6 q

tO -I

(0.

~4 ¢0

a

~6 a I --Q

1

I 20

I 40

I 60

I 80

I 100

I 120

140

Time (min) Fig. 5. Donor reservoir pH during lidocaine iontophoresis. Data were taken during the experiment reported in Fig. 4.

cationic or positively charged drug, is to be delivered. (Similarly, an anionic or negatively charged, drug is delivered from a reservoir containing a cathode.) At first glance, the type of electrode (anode or cathode) used may seem like a trivial detail. However, the material used as the electrode, together with the types of ions in the aqueous phase, will dictate the electrochemical reactions t which occur at the electrode. In general there are two types of electrode materials: those that do not participate in (or are not consumed during) the electrochemistry and those that do. The experimental system shown in Fig. 3, which resulted in the data in Fig. 4, shows a material of the first type, platinum, which does not participate in the electrochemistry. Inert electrode materials such as this force the water in the reservoir to become the fuel for the electrochemistry. Specifically, the electrochemical reaction is (at the anode)

[]5]: 2H20--*O2T + 4H + + 4 e where two molecules of water are broken down into one molecule of oxygen and four hydrogen ions. The equation is balanced by the appearance of four electrons in the metallic phase. Other common inert electrode materials which result in this same anode electrochemistry are carbon, graphite and some types of stainless steel. Knowing that this electrochemistry occurs during the experiment described t The electrochemical reaction occurs to permit the electrons flowing in the wires to exchange with ions in solution. The exchange is quantitative - the charge exchanged at the electrode in the form of electrons is equal to the charge in the form of ions. At the anode, ions lose their charge giving rise to electrons in the anode; similarly, at the cathode ions gain their negative charge from electrons leaving the cathode.

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above (which yields the data in Fig. 4) leads directly to the explanation of a low and time-dependent lidocaine flux. As the experiment continues, more and more hydrogen ions are introduced into the donor solution. These ions compete § with the lidocaine ion (and other ions; see Fig. 1) for current, resulting in a decreasingly lower flux of lidocaine. To verify that this is the case, one need only monitor the pH of the donor solution during the experiment. As is shown in Fig. 5, the pH rapidly falls with time, confirming the appearance of the hydrogen ion in the donor reservoir. Clearly it is the presence of the hydrogen ion which causes the steadily decreasing lidocaine flux. One might conclude that this problem could be easily solved by adding a buffer to the donor formulation. While it does prevent the drop in pH, it does little to change the lidocaine flux, since the buffer ions now compete for the current instead of the hydrogen ions. Replacing the inert electrode material with a material which participates in or is consumed by the electrochemistry provides an elegant solution to this problem because the use of water in the electrochemistry is avoided [17,18]. Thus no hydrogen ions are generated. There are many electrochemistries which avoid water hydrolysis. The key criteria for selecting one are: - the potential at which the electrochemistry takes place is below 1.22 V, the oxidation potential of water; - the ionic species required for the reaction are available in the quantities needed. An example of one particularly elegant electrochemistry for the example of lidocaine iontophoresis is this one [11,16]: Ag + CI - ---}AgCI + e This electrochemistry may be selected by simply replacing the platinum electrode in the above example with silver. When the original experiment is repeated, the data shown in Fig. 6 result. The higher and steady lidocaine transport demonstrates the absence of electrochemical generation of competing ions. There are two other reasons why silver is a particularly elegant choice for lidocaine hydrochloride or any other hydrohalide salt. In solution, the hydrohalide salt form of the drug dissociates into the drug cation and the halide counterion. Thus the drug itself provides one of the ions for the electrochemistry. Further the silver halide which is produced during the reaction is an insoluble salt which precipitates on the electrode. Thus there are no ions generated which compete for the current. The above example relates to electrochemistries at an anode. It is left as an exercise for the reader to work through the quite analogous situation at a cathode. Note that the ion generated at inert cathodes is the O H - , which can be blocked by electrochemistries which occur at potentials below that of the § It is a requirement that all the transport numbers add to 1. Thus the introduction of a new highly mobile ion, H ÷, results in less current being transported by the lidocaine.

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7 6

~) ~.~ 4

~3

E=7%

1

2

3

4

time (h) Fig. 6. Lidocaine flux using an in vitro excised skin experimental system with active electrodes. Experimental conditions identical to those in the experiment reported in Fig. 4.

reduction of water. For a complete discussion of electrochemistries for iontophoresis, see Phipps and Untereker [17] or Petelenz [18].

111.4. The Franz configuration of the in vitro experimental system The common thread running through this chapter is the evolution of a model system which predicts results obtainable in vivo. So far we have seen the preference for electrochemistries which avoid the generation of parasitic ions. This second point, the preference for the so-called Franz configuration of an in vitro system shown in Fig. 7, is made for two reasons. The first is that in the side-by-side configuration only drug formulation, that is, the drug in its solvent with other agents as deemed appropriate, may be studied. While formulation is a significant variable, eventually the formulation must be placed in a device which can be placed on an animal. This introduces a new set of problems, such as prevention of leaks and evaporation, access for electrical leads, etc. Consideration of these problems leads directly to containment means such as gel formulations and a new range of formulation variables. It is beyond the scope of this article to cover the transition from formulations which may be studied in the side-by-side system to devices that can be placed on animals which embody usable formulations. In order to obtain results in vivo, however, this has to be done. The second reason for a preference for the Franz configuration is an economic one. In order to make these experimental systems handleable and to cover a reasonable surface area of skin, the reservoir volumes are usually sizable - on the order of a few to several ml. It is also true that the deliverable drug flux is proportional to the molar concentration of the drug in the

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reservoir, with higher concentrations providing higher fluxes up to a point. For many of the new emerging biotech drugs, for which iontophoresis is a desirable delivery technology, large volumes at high concentrations can quickly consume large amounts of expensive compound. Devices, however, can be made which have reasonable surface areas yet modest volumes. Such devices, easily studied with the Franz configuration, provide a relatively inexpensive system for studying these compounds. 111.5. In vitro perfused systems For passive transdermal delivery of drugs, there is ample evidence that the stratum corneum is the rate-limiting barrier to the drug flux [19,20]. Since that barrier is intact on excised skin, it is expected that passive fluxes observed in vitro using excised skin would be predictive of passive fluxes in vivo. For many compounds this is indeed the case. It would be easy to assume that in the case of iontophoresis this same barrier, the stratum corneum, would be the ratelimiting barrier. But there are many differences between diffusion powered passive transport and electric-field-driven active transport #. One difference is that iontophoresis requires that the molecule to be transported be charged. In general, charged molecules are hydrophilic and poorly soluble in lipid phases. Experience has shown that molecules that easily diffuse through the stratum corneum are highly lipophilic. Thus ionic species have difficulty partitioning into the stratum corneum. Perhaps the biggest difference between passive and iontophoretic transport is the driving force. Concentration-dependent diffusion gradients impart a small energy to the molecule compared to that imposed by an electric potential gradient. This is amply demonstrated by the dramatic difference in lag time - the time it takes for a given level of transport through # Electric-field-driven transport has two components: one which acts on charged particles and another which acts on both charged and uncharged particles. Transport resulting from the force of the electric field on the particle is called iontophoresis. Transport resulting from the movement of the particle's solvent due to the force of the electric field on other charged particles is called electroosmosis or electroendosmosis.

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the membrane to be established - between the two methods. In iontophoresis, lag times are measured in minutes (see Fig. 10). In passive diffusion, they are much longer and frequently measured in hours. These very short lag times raised many questions about the nature of a barrier which could be breached so swiftly and led to in vivo experiments to study the concentration of the charged molecule as a function of depth into the skin. It is generally believed that the highest concentration would be associated with the rate-limiting barrier. Thus, if the stratum corneum were the ratelimiting barrier, the highest concentrations would appear there. The results of these experiments, reported in detail elsewhere [21], are shown in Fig. 8 for the compound lidocaine hydrochloride. These experiments repeatedly show the highest concentrations at depths between 0.1 and 2 mm and very little compound in the stratum corneum.* For iontophoresis, then, transport into the skin is much different than for diffusion. The image we have is one where the large electric field propels the molecule down whatever aqueous pathways exist - its motion even less impeded by the fact that it cannot partition into the abundant lipid moieties along the way. After passing through the stratum corneum, the slower moving * These data imply that in iontophoresis, the stratum corneum may not be the rate-limiting barrier. This question was addressed using lidocaine comparing stripped (Scotch tape method) and unstripped skin. In our hands, the higher flux was obtained with unstripped skin.

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molecules accumulate in the dermis below the stratum corneum where they are absorbed by the blood stream and distributed to the rest of the animal's body. Given our belief that the highest concentrations would be associated with the rate-limiting barrier, the hypothesis quickly formed that the interface between the dermal tissue and the microcirculatory system was the rate-limiting barrier. This hypothesis was tested [21] by coiontophoresing a vasoconstrictor (norepinephrine) and a vasodilator (tolazoline) with the radio-labelled lidocaine. These results are shown in Fig. 9. These results reinforce this hypothesis, since the presence of a vasoconstrictor results in higher dermal concentrations of lidocaine and the presence of a vasodilator results in lower dermal concentrations of lidocaine. These results implying that the stratum corneum is not the rate-limiting barrier raised very serious questions about the validity of in vitro excised skin experimental models for studying iontophoretic transport. The biggest concern was that the in vitro excised skin system would over-estimate the in vivo transport, since it lacked the anatomical structure now believed to be the ratelimiting barrier. Clearly, what was needed was an experimental model which included all of the anatomical structure of the skin, including vasculature, but lacked the large volume of distribution and the rapid clearance of the whole animal +. Of the two such models available, the Krueger rat model [22] and the

+ Whole-animal studies are obvious as well. However, given the large volume of distribution and the rapid clearance of [idocaine and our expectation of low current efficiency, 200 cm 2 patches were calculated to give a good change of measurable serum levels. Skin flap experiments were easier•

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Riviere porcine skin flap model [23,24], the skin flap model was preferred primarily because it could accommodate larger electrode systems, provided larger volumes of perfusate to assay for the drug and it avoided in vivo metabolic processes. Studies of lidocaine iontophoresis using the porcine skin flap are published elsewhere [25]. These studies probe the variables relevant to iontophoretic transport and document that the three most significant variables for drug flux are: - current level; - molar concentration of drug; - molar concentration of vasodilator. Of importance here is the level of flux which can be achieved in the skin flap under conditions similar to those studied using the excised skin model. Results for four replicate skin flaps, which contain an optimum formulation of drug and vasodilator, are shown in Fig. 10. Several key conclusions can be reached from these data and the data shown in Fig. 11 which compare the flux from three variations of in vitro model systems under identical conditions: the lidocaine flux in the skin flap is unexpectedly high. Rather than being less than in the in vitro excised skin model, skin flap flux exceeds excised skin flux by a factor of three; -

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- as observed using the in vitro excised skin model, the reproducibility of the flux is good; also, as observed in the in vitro excised skin model, lag times are surprisingly short; while the nearly steady-state flux is higher in the skin flap model, more time is required to reach this steady state. -

111.6. In vivo results

Fig. 11, which compares the current efficiency of lidocaine in three different in vitro experimental systems offers two reasonable predictions for in vivo results. Given that the skin flap results are higher than the excised skin results, the reservoir systems no longer need to be enormous to achieve detectable serum levels. Thus the key test, the in vivo test, can be performed to determine which of these two predictions, curve B or curve C in Fig. 11, more closely approximates actual in vivo serum levels. This test was performed in the following way. Two weanling Yorkshire swine, about 20 kg each, were sedated with ketamine and given an i.v. bolus of lidocaine. A series of blood specimens were drawn over the next several hours and assayed for lidocaine. From the serum levels of lidocaine and the mass of lidocaine injected, both the volume of distribution and the clearance for each pig can be calculated [26]. To predict serum levels resulting from lidocaine iontophoresis, infusion rates represented by curve B and curve C in Fig. 11 were assumed. These infusion rates, when combined using the appropriate pharmacokinetic model and the

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distribution volume and clearance of each pig, provide the predicted time course of the serum level of lidocaine in each pig. The next step was to perform the actual iontophoresis experiment, draw blood during the course of the experiment and assay the blood for lidocaine. As a result of the modeling, 50 cm 2 reservoir systems were prepared: one for the lidocaine plus vasodilator formulation (anode) and one for saline (cathode). A current of 10 mA was used for 4 h. Based on the data from Fig. 11, the lidocaine flux to the animal was projected to be 300 ~tg/min (curve C) or 100 ~tg/ min (curve B). The results of this experiment are shown in Fig. 12. Only one predicted curve, that for the skin flap model, is shown because of the excellent fit of the serum levels during iontophoresis with that predicted curve. Thus, in answer to the key question, it is the skin flap model which is predictive of in vivo results. A similar ability of the porcine skin flap model to predict in vivo iontophoretic drug delivery in humans has recently been reported [27]. An intriguing question remains. How can the natural system, which includes

MODEL SYSTEMS IN IONTOPHORESIS - TRANSPORT

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285

the rate-limiting barrier, be more efficient than the excised system which does not include the natural rate-limiting barrier? The answer to this question remains to be fully answered. However, recent experiments point in an interesting direction. The measured difference is one of current efficiency (or transport numbers), which is the ratio of the current transporting lidocaine to the total current flowing. The total current flowing includes all the non-drug ions which, in the in vitro excised skin experimental system, includes the receptor reservoir ions. In general, the component of current due to any ionic species is proportional to its concentration and mobility [28]. Thus changing the saline concentration in the receptor reservoir of the in vitro excised skin system alters the current component due to receptor solution ions. Since the sum of the transport numbers must add to unity, lowering the receptor ion transport number will increase the drug transport number. An interesting aspect of this is the leverage involved. If the drug transport number is 0.1, then the combined receptor reservoir ion transport number must be 0.9. If the combined receptor reservoir ion transport changes by only 10%, say from 0.9 to 0.8, the drug transport number must double to 0.2. In vivo, there is little that can be done to alter the concentrations and mobilities of 'receptor' ions. Thus, the difference between skin flap and in vitro excised skin drug transport may lie with the concentrations and mobilities of the ionic species which participate as the dermal current. At this point it is illustrative to return to the Dilaudid T M example in section B2, above. Since this is a hypothetical example, suppose that the results which have been achieved for lidocaine where achieved for Dilaudid TM. The in vitro excised skin experimental system predicts a current efficiency of 7% which, through Fig. 2, implies an anode area of 40 cm2 and a device area approaching 100 cm 2 if the current density cannot be increased above 200 ~tA/cm2. It is doubtful if such a large system could be successfully commercialized even for non-invasive patient-controlled pain management. However, a current efficiency of 22% implies an anode area of 13 cm 2 and a total device area of a little over 30 cm 2 or just a little larger than a 2 " x 2" gauge pad. The commercial prospects for such a system are much brighter. IV. Conclusions

The utility of any model system used to predict results of a process which cannot be directly studied is proportional to the accuracy of the predictions of that model. In the preceding paragraphs, systems thought to model drug delivery by iontophoresis have been reviewed with the goal of determining which model system, if any, is most predictive. The conclusion reached here is that the skin flap model is most predictive. This has been documented in detail for lidocaine; results not reported here show similar results with other drugs, including peptides. When the goal of iontophoresis experimentation is to determine serum drug levels which can be obtained by iontophoresis, several recommendations can be

286

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made which will avoid confusing results and/or lead to underestimates of the achievable drug levels. These are: - use a constant current power source. Drug flux is proportional to current the use of other sources such as constant voltage sources results in a timedependent current (and hence time-dependent transport numbers) and introduces an unnecessary layer of data manipulation which provides opportunity for misinterpretation of results. - s e l e c t an appropriate electrochemistry and insure that the desired electrochemistry occurs at all times during iontophoresis. There are a surprising number of alternate electrochemistries which can introduce unwanted ions to the detriment of drug delivery. These will occur if the designed conditions are not maintained. - study dosage forms with the appropriate formulation, not just formulations. The nature of the dosage form can influence both the delivery of the drug and efficient use of the compound. - avoid model systems which use excised skin. In these systems it is difficult to provide a receptor reservoir which has electrical properties equivalent to skin, and it is not possible to study the role dermal vasculature plays in drug uptake. For the study of drug transport from the outside of the stratum corneum to the systemic circulation, all tissues which the drug ion will encounter on its journey should be present. The degree to which the excised skin experimental systems underestimate the achievable drug flux is sometimes surprisingly large and can lead to an erroneous negative conclusion about the commercial feasibility of an iontophoresis dosage form, as in the hypothetical case of Dilaudid TM.

References 1 Banga, A.K. and Chien, Y.W. (1988) lontophoretic delivery of drugs: fundamentals, developments and biomedical applications, J. Controlled Release 7, 1 24. 2 Leduc, S. (1900) Introduction of medicinal substances into the depth of tissues by electric current, Ann. Electrobiol. 3, 545 560. 3 Inchly, O. (1921) The influence of the electric current on the absorption of drugs, J. Pharm. Exp. Ther. 18, 241-256. 4 Cumberbatch, E.P. (1933) In: H. Kimpton (Ed.), Essentials of Medical Electricity, London. 5 Bellantone, N.H., Rim, S., Francoeur, M.L. and Rasadi, B. (1986) Enhanced percutaneous absorption via iontophoresis, i. Evaluation of an in vitro system and transport of model compounds, Int. J. Pharm. 30, 63-72. 6 Coyle, N. Mauskop, A., Maggard, J. and Foley, K.M. (1986) Continuous subcutaneous infusions of opiates in cancer patients with pain, Oncol. Nurs. Forum 13, 53 57. 7 Singh, J. and Roberts, M.S. (1989) Transdermal delivery of drugs by iontophoresis: a review, Drug Design Deliv. 4, 1-12. 8 Tyle, P. (1986) Iontophoretic devices for drug delivery, Pharm. Res. 3, 318-326. 9 Gangarosa, L.P. (1981) Defining a practical solution for iontophoretic local anesthesia of skin, Methods Find. Exptl. Clin. Pharmacol. 3, 83 94. 10 Rapperport, A.S., Larson, D.L., Henges, B.A., Lynch, J.B., Blocker, T.G. and Lewis, R.S. (1965) Iontophoresis: a method of antibiotic administration in the burn patient, Plast. Reconstr. Surg. 36, 547 552.

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11 Haase, R. and Harff, K. (1983) On electroosmosis and related phenomena, J. Membrane Sci. 12, 279-288. 12 OKabe, K., Yamaguchi, H. and Kawai, Y. (1986) New iontophoretic transdermal administration of the beta blocker metoprolol, J. Controlled Release 4, 79 85. 13 Slough, C.L., Spinelli, M.J. and Kasting, G.B. (1988) Transdermal delivery of etidronate (EHDP) in the pig via iontophoresis, J. Membr. Sci. 35, 161 165. 14 Wearley, L., Liu, J.C. and Chien, Y.W. (1989) Iontophoresis facilitated transdermal delivery of verapamil. I. In vitro evaluation and mechanistic studies, J. Controlled Release 8, 237-250. 15 Latimer, W.M. (1952) The Oxidation State Of The Elements And Their Potentials In Aqueous Solutions, Prentice Hall, New York, Ch. 3 and 4. 16 Ferris, C.D. (1977) Introduction to Bioelectrodes, Plenum, New York, pp. 90-95. 17 Phipps, J.B. and Untereker, D.F., U.S. Patent 4,747,819. 18 Petelenz, T.J., Stephen R.L. and Jacobsen, S.C., U.S. Patent 4,752,285. 19 Waiters, K.A. (1986) Percutaneous absorption and transdermal therapy, Pharm. Technol. 10, 30-46. 20 Weichers, J.W. (1989) The barrier function of the skin in relation to percutaneous absorption of drugs, Pharm. Weekbl. 11, 185-198. 21 Riviere, J.E., Monteiro-Riviere, N.A., Inman, A.O., Determination of lidocaine concentrations in skin after transdermal iontophoresis: effects of vasoactive drugs, Pharm. Res., in press. 22 Pershing, L.K. and Krueger, G.G. (1989) Human skin sandwich flap model for percutaneous absorption. In: R.L. Bronaugh and H.I. Maibach (Eds.), Percutaneous Absorption, Dekker, New York, Ch. 24, 397-414. 23 Riviere, J.E., Bowman, K.F., Montiero-Riviere, N.A., Dix, L.P. and Carver, M.P. (1986) The isolated perfused porcine skin flap (IPPSF). I. A novel in vitro model for percutaneous absorption and cutaneous toxicology studies, Fund. Appl. Toxicol. 7, 444453. 24 Monteiro-Riviere, N.A., Bowman, K.F., Scheidt, V.J., Riviere, J.E. (1987) The isolated perfused porcine skin flap (IPPSF). II. Ultrastructural and histological characterization of epidermal viability, In Vitro Toxicol. 1,241 252. 25 Riviere, J.E., Sage, B. and Williams, P.L. (1991) Effects of vasoactive drugs on transdermal lidocaine iontophoresis, J. Pharm. Sci. 80, 615-620. 26 Goldstein, A., Aronow, L. and Kalman, S.M. (1974) Principles of Drug Action: the Basis of Pharmacology, Wiley, New York, 21(~219. 27 Riviere, J.E., Williams, P.L., Hillman, R. and Mishky, L., Quantitative prediction of transdermal iontophoretic drug delivery of arbutamine in humans using the in vitro isolated perfused porcine skin flap (IPPSF), J. Pharm. Sci., in press. 28 Bookris, J. O'M. and Reddy, A.K.N. (1977) Modern Electrochemistry, Plenum, New York, Vol. 1,345-373.