Mass Transport Analysis of the Enhanced Buffer Capacity of the

Article Cite This: Mol. Pharmaceutics XXXX, XXX, XXX−XXX

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Mass Transport Analysis of the Enhanced Buffer Capacity of the Bicarbonate−CO2 Buffer in a Phase-Heterogenous System: Physiological and Pharmaceutical Significance Jozef Al-Gousous

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University of Michigan, College of Pharmacy, Department of Pharmaceutical Sciences, 428 Church Street, Room 4002, Ann Arbor, Michigan 48109, United States

Kathy X. Sun University of Michigan, College of Pharmacy, Department of Pharmaceutical Sciences, 428 Church Street, Room 4002, Ann Arbor, Michigan 48109, United States

Daniel P. McNamara Drug Product Science and Technology, Bristol-Myers Squibb, 1 Squibb Drive, New Brunswick, New Jersey 08903, United States

Bart Hens University of Michigan, College of Pharmacy, Department of Pharmaceutical Sciences, 428 Church Street, Room 4002, Ann Arbor, Michigan 48109, United States

Niloufar Salehi University of Michigan, College of Pharmacy, Department of Pharmaceutical Sciences, 428 Church Street, Room 4002, Ann Arbor, Michigan 48109, United States

Peter Langguth Johannes Gutenberg Universität Mainz, Fachbereich Chemie Pharmazie und Geowissenschaften, Department of Biopharmaceutics and Pharmaceutical Technology, D-55099 Mainz, Germany

Marival Bermejo Universidad Miguel Hernández, Ingenieria: Area Farmacia, Ctra. Alicante-Valencia N 332, 03550 Sant Joan d’Alacant, Spain

Gregory E. Amidon University of Michigan, College of Pharmacy, Department of Pharmaceutical Sciences, 428 Church Street, Room 4002, Ann Arbor, Michigan 48109, United States

Gordon L. Amidon* University of Michigan, College of Pharmacy, Department of Pharmaceutical Sciences, 428 Church Street, Room 4002, Ann Arbor, Michigan 48109, United States S Supporting Information *

Received: July 24, 2018 Revised: September 28, 2018 Accepted: October 10, 2018

© XXXX American Chemical Society

A

DOI: 10.1021/acs.molpharmaceut.8b00783 Mol. Pharmaceutics XXXX, XXX, XXX−XXX

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Molecular Pharmaceutics

ABSTRACT: The bicarbonate buffer capacity is usually considered in a phasehomogeneous system, at equilibrium, with no CO2 transfer between the liquid buffer phase and another phase. However, typically, an in vitro bicarbonate buffer-based system is a phase-heterogeneous system, as it entails continuously sparging (bubbling) the dissolution medium with CO2 in a gas mixture, at constant ratio, to maintain a constant partial pressure of CO2 (g) and CO2(aq) molarity at a prescribed value, with CO2 diffusing freely between the gas and the aqueous phases. The human gastrointestinal tract is also a phase-heterogeneous system, with CO2 diffusing across the mucosal membrane into the mesenteric arterial blood, which serves as a sink for CO2 from the intestinal lumen. In this report, a mass transport analysis of the apparent buffer capacity of a phase-heterogeneous bicarbonate−CO2 system is developed. It is shown that, most significantly, a phase-heterogeneous bicarbonate−CO2 system can have a much higher buffer capacity than a phase-homogeneous system such that the buffer capacity is dependent on the bicarbonate concentration. It is double that of a phase-homogeneous system at the pH = pKa for a monoprotic buffer at the same concentration. This buffer capacity enhancement increases hyperbolically with pH above the pKa, thus providing a much stronger buffering to keep the pH in the physiologically neutral range. The buffer capacity will be dependent on the bicarbonate molarity (which in vivo will depend on the bicarbonate secretion rate) and not the pH of the luminal fluid. Further, there is no conjugate acid accumulation as a result of bicarbonate neutralization, since the resulting carbonic acid (H2CO3) rapidly dehydrates producing CO2 and H2O. The mass transport analysis developed in this report is further supported by in vitro experimental results. This enhanced bicarbonate buffer capacity in a phaseheterogeneous system is of physiological significance as well as significant for the dissolution and absorption of ionizable drugs. KEYWORDS: CO2, bicarbonate, buffer capacity, acid and base dissolution, phase-heterogeneous, in vivo gastrointestinal buffering buffer phase. They did not evaluate the buffer capacity for the effect of the carbonic acid dehydration with the resulting movement of the generated CO2 out of the liquid as would occur in a phase-heterogeneous system, where CO2 transfer into and out of the aqueous buffer phase takes place. And, actually, the in vitro bicarbonate dissolution system used is often not a phase-homogeneous system as CO2 is typically continuously pumped (sparged) into the medium maintaining a constant CO2 concentration in both aqueous and gas phases, thus allowing CO2 to diffuse between two different phases (the gaseous and the aqueous). The human small intestine is clearly a phase-heterogeneous system, since bicarbonate (HCO3−) is secreted into the intestine and carbon dioxide readily permeates through cell membranes,3 thus leaving the intestinal lumen to the blood and ultimately being exhaled. As a result, the buffer capacity of the bulk solution will not follow the typical buffer capacity (van Slyke)4 equation. This is of particular importance in situations where the dissolution of an ionizable solute (acidic or basic drug) would alter the bulk pH, as in the case of a relatively low buffer capacity fluid such as the fluid in the intestinal lumen.5 Actually the significance of this phase-heterogeneous nature of typical bicarbonate buffers was noticed by McNamara et al.,6 who observed that pH 5 bicarbonate buffers managed to enhance dipyridamole dissolution compared to a nonbuffered medium despite the very low bicarbonate carbonic acid molarities and attributed that to them being supplied by CO2 diffusion from the gas phase. In this report we developed an expression for the buffer capacity of a phase-heterogeneous bicarbonate buffer system where the mass transfer of carbon dioxide from the solution phase is allowed as in an in vitro sparged setup and in the human intestine in vivo. The actual buffer capacity of the small intestine results from the secretion of bicarbonate into the intestine and the resultant protonation of HCO3−, followed by dehydration of H2CO3 resulting in CO2 which moves out of the system, thus limiting accumulation of conjugate acid (H2CO3). The mass transport analysis summary presented below (see Supporting Information for more details) is for a phase-heterogeneous system which exhibits a substantially enhanced buffer capacity of

1. INTRODUCTION The human small intestinal fluid is buffered chiefly by bicarbonate while in vitro dissolution testing methods typically employed in the pharmaceutical industry are based on phosphate or acetate buffers. Bicarbonate buffers are somewhat challenging for routine laboratory dissolution testing owing to the tendency of carbon dioxide to leave aqueous solution and to form gas bubbles. Bicarbonate buffers in vitro are often continuously sparged with carbon dioxide at a fixed percent, in order to maintain the solution CO2(aq) concentration in an equilibrium (following Henry’s law) and thus pH. This, however, introduces the potential for gas bubbles to form in the medium, complicating the dissolution process. Therefore, instead of bicarbonate, simpler buffers like phosphate and acetate are typically employed in compendial dissolution media. However, there is additional complexity due to the hydration (kh) and dehydration (kd) reaction rate constants of CO2 and H2CO3 in aqueous solutions in the absence of carbonic anhydrase. These hydration and dehydration reaction rates are somewhat larger (dehydration) and smaller (hydration) than the diffusional rate in the solid−liquid boundary layer, thus impacting the dissolution rate.1

It has been shown by Krieg et al.1,2 that a mass transfer model assuming an irreversible dehydration reaction (the dehydration rate is about ∼500 times faster than the hydration rate) could approximately account for the lack of equilibration between CO2 and H2CO3 in the boundary layer during dissolution. This approximate model yielded improved predictions for the intrinsic dissolution rates of several acidic drugs in bicarbonate buffers. Transport models developed based on “ordinary” equilibrium buffer behavior gave significantly less accurate predictions.1,2 The usual transport models assume a constant total buffer molarity (i.e., that of the conjugate base plus that of the conjugate acid) in the bulk solution i.e. a phase-homogeneous system behavior with no CO2 transfer into or out of the aqueous B


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Figure 1. Buffer capacity enhancement factor (ε) vs pH for a phase-heterogeneous bicarbonate−CO2 buffer system resisting pH decrease vs pH.

the bicarbonate buffer over that of a phase-homogeneous system, thus providing a much stronger buffer, maintaining the solution pH near neutrality. This analysis is supported by experimental results also presented below.

transfer of CO2 between the gaseous and the aqueous phases), the buffer capacity is equal to

2. BUFFER CAPACITY EXPRESSION FOR THE BICARBONATE BUFFER IN A PHASE-HETEROGENEOUS SYSTEM Gilbert derived an expression for the buffer capacity of a phaseheterogeneous bicarbonate−CO2 buffer system in the blood.7 However, that derivation is based on the linear relationship between the bicarbonate molarity and pH observed for blood plasma over a very narrow pH range,7,8 and can be valid only for blood plasma in contact with red blood cells and over the extremely narrow pH range of human blood.6 Therefore, a more general expression is needed. We derived a more general expression below (see Supporting Information). We have adapted the derivation of the van Slyke equation4 to an acid-titrated phase-heterogeneous system by assuming it to be well-sparged and well-mixed with CO2. The sparging is assumed to result in a very rapid transfer of CO2 between the gaseous and liquid phases and very rapid elimination of any generated gaseous CO2 away from the vicinity of the liquid (i.e., essentially a constant molarity of dissolved CO2 (aq) with any additional dissolved gas generated by the neutralization of bicarbonate being rapidly volatilized and not altering the solid surface (interfacial) contact with the dissolving fluid). The result of this analysis (see Supporting Information) is that the buffer capacity of a bicarbonate buffer in a phase heterogeneous system (βp) is dependent on bicarbonate concentration:

Taking into account, at the intestinal pH values, the concentrations of [H+], [OH−], and [CO32−] can be considered negligible relative to the concentration of [HCO3−], this equation reduces to eq 1. This result indicates that the buffer capacity is a function of solely the bicarbonate molarity and NOT the pH (though the target pH value will impose a ceiling on the maximum bicarbonate molarity, and so buffer capacity that is reachable in a normal experimental setup where the maximal pCO2 that can be reached does not exceed 1 atm). Moreover, it will always be larger than that of a phase-homogeneous system (β), with the buffer capacity enhancement factor (ε) being (see Supporting Information for the proof):

βp = 2.303[HCO−3 ]

βp = 2.303{[H+] + [OH−] + [HCO−3 ] + 4[CO32 −]}

ε = βp /β = 1 +

Ka [H+]

(2)

(3)

Equation 2 implies that, at pH = pKa, a phase-heterogeneous CO2/bicarbonate system will have double the buffer capacity of a phase-homogeneous system at the same molar concentration. Figure 1 shows a graph of the buffer capacity enhancement (ε) against pH based on eq 2 and indicates that ε increases with increasing pH, being approximately 10% at pH 5 rising to 100% at pH 6.04 (equal to the pKa of bicarbonate in a phasehomogeneous aqueous system at physiological temperature and ionic strength1) and then rising very steeply, hyperbolically, above the pKa. Note that the pKa in eq 2 and Figure 2 is 6.04, the apparent pKa of bicarbonate in bulk solution. The intrinsic pKa of carbonic acid (H2CO3) in the boundary layer is 3.55 and is the pKa value in the diffusional boundary layer mass transport modeling.1 The physiological significance of this strong increase in buffer capacity enhancement above the bicarbonate pKa (= 6.04 in a phase-homogeneous aqueous system with CO2 in equilibrium)1

(1)

This is in agreement with the expression that was derived by Butler9 for the scenario of base addition using a similar approach, where he showed that for a bicarbonate buffer with a constant dissolved CO2 molarity (implying instantaneous mass C


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Figure 2. A diagram showing the three setups where the influence of the CO2 mass transfer rate and acid addition rate on the buffer capacity was investigated.

is that the fluid in the intestinal lumen is strongly buffered by bicarbonate, even at pH values lying farther than one pH unit from its apparent pKa (e.g., in the distal small intestine10). The experimental verification is presented below.

rotation speed of 50 rpm. In these experiments, the initial pH was maintained at 7.1−7.2, and the gas was introduced without using the sinker frits. In one configuration (Setup A), at the start of the titration, the tube (opening diameter = 4 mm) was moved up so that its opening only touched the surface of the liquid, and the sparging was done at a flow rate of 200 mL/min air and 20 mL/min CO2. At 3 min intervals, the pH was recorded and 1 mL of 1 M HCl was then added. In the second configuration (Setup B), the tube was kept at a 5 cm depth in the medium, and sparging was done at 500 mL/min air and 50 mL/min CO2, so resulting in a faster CO2 mass transfer rate. Recording of pH and addition of HCl were also done every 3 min. The third configuration (Setup C) differed from the second by adding the HCl and measuring the pH at 7 min intervals instead of 3 min intervals (i.e., slower acid input). In all of the three setups, the total volumes did not change by more than 2% over the course of the titrations. These titrations were also performed in duplicate. Figure 1 summarizes the three setups. The buffer capacity was calculated as the amount of acid added per unit buffer volume per unit pH change. 3.3. Determination of CO2 Mass Transfer Kinetics. The mass transfer kinetics of CO2 in the Setups A, B, and C discussed above were evaluated. After bringing a 20 mM bicarbonate buffer (ionic strength adjusted to 0.15 M with NaCl) to near saturation with pure CO2, the sparging conditions of the three setups were reproduced except for using only air (at equal total gas flow rates) as the sparging gas. The pH change over time was monitored. The experiments were performed in duplicate.

3. EXPERIMENTAL SECTION 3.1. Materials. Carbon dioxide was obtained from Metro Welding, MI, USA. All other materials used were of analytical grade. 3.2. Buffer Capacity Determination. A 10 mM bicarbonate buffer (no NaCl added) was set up by dissolving the appropriate amount of sodium carbonate in 900 mL of deionized water and sparging it with a mixture of compressed air and carbon dioxide (Metro Welding, MI, USA) in a single jacketed dissolution vessel stirred with a paddle at 100 rpm. The flow rates of air and carbon dioxide were 490 and 17.5 mL/min respectively and were controlled by gas flow controllers (King Instrument Company, CA, USA). The gases entered the solution through HPLC mobile phase sinker frits to provide fine bubbles. The pH and the CO2 concentrations were monitored by means of a pH meter (AR60, PA, USA) and CO2 monitor (YSI 8500, OH, USA). When constant pH and CO2 were achieved, the bicarbonate buffer was titrated with 0.9 mL aliquots of 1 M HCl at 10 min intervals. The pH and CO2 levels were recorded over the 10 min separating the addition of successive HCl aliquots. The buffer capacity of a 15 mM pH 7.3 phosphate buffer (with NaCl added to achieve a total ionic strength of 0.2 M) was also determined by titrating 60 mL of it with 0.06 mL aliquots of 1 M HCl. These titrations were performed at room temperature and in duplicate. The total volumes did not change during the course of the titrations by more than 1%. Another set of three titration experiments was performed for 20 mM bicarbonate solutions (1 L volume), with the ionic strength adjusted to 0.15 M using NaCl, at 37 °C in a USP type II dissolution tester (Hanson Research SR6, USA) at a paddle

4. RESULTS As shown in Figure 3, in contrast to phosphate, the bicarbonate buffer (when sparged using sinker frits and titrated at 10 min intervals) did not exhibit a peak in its buffer capacity at a pH value near to its pKa supporting the assertion of its buffer capacity’s independence of pH. The buffer capacity decreased as the titration progressed due to the decrease in bicarbonate D


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Figure 3. Buffer capacity vs pH for bicarbonate and phosphate buffers (mean ± SD; n = 2).

As for the characterization of the mass transfer rates of CO2 in the three setups, the Henderson−Hasselbalch equation was used to calculate the concentrations of CO2(aq) in the buffer from the pH values over time. The pH profiles were fitted with exponential trend lines to estimate first-order rate transfer constants of CO2 in the different setups, and the resulting r2 values ranged from 0.9929 to 1. An example is shown in Figure 6. The resulting CO2 mass transfer rate constants were 0.00802 ± 0.00040 (mean ± SD) for Setup A and 0.120 ± 0.003 for Setups B and C. These rate constants were used to calculate predicted theoretical buffer−capacity profiles for the titrations using the following equation:

molarity in the solution. When the buffer capacity was plotted as a function of the remaining bicarbonate molarity (assuming complete reaction between the added strong acid and the originally present bicarbonate), a straight line, with a slope very close to the value of 2.303, would be expected from eq 1(Figure 4). When the buffer capacities at each increment of acid added were divided by the remaining bicarbonate molarity, then an averaged value of 2.286 (very close to the theoretical value of 2.303) was obtained as well. When normalized to total buffer molarity, the maximal buffer capacity of the bicarbonate system was approximately triple that of the phosphate (the observed ratio of 2.74 was only 7.5% different from the expected ratio of 2.53). Figure 5 shows how the actual behavior of the bicarbonate system’s buffer capacity is a function of both the acid input rate and the CO2 sparging rate with the faster acid input and the slower CO2 sparging rate combination demonstrating a buffer capacity−pH profile during titration more similar to that of a traditional liquid only buffer (hypothetical profile calculated using the van Slyke equation). A combination of slower acid input and faster CO2 sparging rate gives a buffer capacity−pH profile with a peak that is sharper and shifted toward higher pH values approaching the hypothetical situation where the CO2 equilibration between solution and gas phases is instantaneous (hypothetical profile calculated using eq 1 to calculate buffer capacity, and Henderson−Hasselbalch equation to calculate the corresponding pH), similar to the behavior observed for the equilibrated bicarbonate system in Figure 3. In the equilibrated bicarbonate buffer setup, the acid input is slower and the sparging process is more efficient due to the use of sinker frits which subdivide the gas stream into fine bubbles to speed up the gas−liquid mass transfer. Thus, the sparging rate of CO2, following neutralization of the added acid by bicarbonate, is the key to understanding a heterogeneous system and the larger capacity of bicarbonate to buffer an acid challenge.

βapp =

−ΔC ÄÅ ÉÑ Å ÑÑ | l Å (1 − e−(n − 1)kτ ) o ([HCO−3 ]0 − nΔC) × ÅÅÅÅKHpCO2(g ) + ΔC e−kτ ÑÑÑÑ o o o − τ k o ÅÅÇ ÑÑÖ o (1 − e ) o o ÄÅ ÉÑ logm } − − τ n k ( 1) Å Ñ o o Å Ñ ) −kτ ÑÑ − o ÅÅK pCO + (1 − e o o ΔC e ÑÑ × ([HCO3 ]0 − (n − 1)ΔC) o o ÅÅÅÇÅ H 2(g ) o − τ k ÑÑÖ (1 − e ) n ~ (4)

where βapp is the apparent buffer capacity in mM/pH unit, ΔC is the amount of H+ added per unit volume of solution with each aliquot of the titrant (in mmol/L), n is the total number of the added aliquots of titrant, k is the first-order mass transfer rate constant for CO2 in the system, τ is the interval in minutes between titrant additions, pCO2 (g) is the CO2 partial pressure with which the initial CO2(aq) molarity of the system would be in equilibrium, [HCO−3 ]0 is the initial bicarbonate molarity, and KH is Henry’s constant in mM/atm (with the product KHpCO2 (g)being equal to the initial CO2(aq) molarity which can be calculated using the initial pH from the Henderson− Hasselbalch equation). An overlay of the predicted and observed profiles is shown in Figure 7. The full derivation of eq 4 is shown in the Supporting Information. E


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Figure 4. Buffer capacity vs bicarbonate molarity of a phase-heterogeneous bicarbonate−CO2 buffer system (mean ± SD; n = 2). Since HCl is a strong acid that can be assumed to react completely with bicarbonate, [HCO3−] following each HCl aliquot addition = initial [HCO3−] − cumulative amount of H+ added per unit volume (cumulative change in total solution volume during the experiment does not exceed 1% and can be ignored). For the most appropriate representation of the buffer capacity observed following each aliquot, the midpoint of the bicarbonate molarities before and after HCl aliquot addition was plotted on the x-axis.

by the derived expression for βp. In vivo, in the gastrointestinal tract, this limits accumulation of the conjugate acid (CO2) which helps the intestinal bicarbonate to continuously buffer against HCl from the stomach and against the dissolution of acidic drugs. This buffer capacity enhancement becomes more pronounced as the pH increases. At lower pH values, the dissolved CO2 concentrations are high relative to the bicarbonate molarity, and, accordingly, they are high relative to the amount of CO2 generated by the acid reacting with bicarbonate. Therefore, it will make less of a difference whether the generated CO2 stays in the system or diffuses away. On the other hand, when the concentrations of dissolved CO2 are low relative to those of bicarbonate, then it will make a larger difference whether the generated CO2 stays in or leaves the fluid. In a hypothetical ideally well-sparged and mixed medium, the restoration of initial pCO2 would be instantaneous; however, the system we are dealing with is not ideal in this regard. The mass transfer rate of dissolved CO2 is finite. Therefore, the actual buffer capacity for the added acid will be a function of the acid addition rate (reflective of the resultant dissolved CO2 generation rate) and the CO2 mass transfer rate. This is clearly shown by Figure 5, where a combination of faster acid input rate and slower CO2 mass transfer rate causes the system’s behavior to approach that of a nonvolatile buffer (since there is more accumulation of neutralization reaction-generated dissolved CO2 in the system). The alternative combination of slower acid input rate and faster CO2 mass transfer rate leads to the system

5. DISCUSSION 5.1. Effect of CO2 Mass Transfer on the Buffer Capacity of Bicarbonate Buffers. When a bicarbonate solution in a phase-heterogeneous system is continuously sparged and mixed with a CO2−air mixture, a steady state is eventually reached where CO2 input = CO2 output, thus resulting in constant pH and pCO2 values. The addition of acid results in the conversion of bicarbonate to carbonic acid which readily dehydrates into CO2 inducing a transient rise in pCO2. However, the continuous sparging and mass transfer of CO2 allows the rapid re-establishment of the initial pCO2. Therefore, the net decrease in the concentration of the bicarbonate (the conjugate base) is not accompanied by a corresponding rise in the level of dissolved CO2 (the effective conjugate acid in bulk) due to its mass transfer into the atmosphere. This is in contrast to a “normal” nonvolatile buffer such as phosphate where the addition of acid results in the conversion of a conjugate base to a nonvolatile conjugate acid that remains in the system, and therefore the pH decrease is caused by both a rise in the conjugate acid concentration and a drop in the conjugate base concentration. Consequently, the bicarbonate buffer, when in a phaseheterogeneous well-sparged and well-mixed system, exhibits an inherently greater capacity to resist changes in pH compared to the other nonvolatile buffers. Furthermore, with the conjugate acid, effectively CO2, escaping and the addition of bicarbonate in a phase-heterogeneous system, the bicarbonate molarity becomes the sole buffer capacity-determining factor as shown F


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Figure 5. Buffer capacity vs pH profile for setups combing different different sparging and acid addition rates (mean ± SD; n = 2). For the exact meaning of Setup A, B, and C, the reader is referred to the Experimental Section of this article.

based on the first-order interphase mass transfer kinetics on which eq 4 is based):

exhibiting a behavior closer to the one associated with a wellsparged phase-heterogeneous system. In the case of acidic drug dissolution in vitro, the drug dissolution rate will reflect the acid addition rate while the dissolution experimental setup design will determine the CO2 mass transfer rate. The reason behind the abrupt rise in the buffer capacity followed by a fall in Setups B and C is that, initially, as acid is added, the CO2 (aq) formed from the reaction between the added acid and bicarbonate results in a rise in the CO2 (aq) concentration. However, as more CO2 (aq) accumulates, the rate of the net transfer of CO2 into the gaseous phase also increases and, therefore, the accumulation of CO2 (aq) levels off with a progressively larger portion of the CO2 (aq) generated managing to move into the gas phase before the subsequent titrant addition. This reduced CO2(aq) accumulation results in the apparent buffer capacity rising; however, it is counteracted by the decrease in bicarbonate molarity as a result of the neutralization of bicarbonate, and at some point this effect becomes predominant resulting in a peak. The accumulation of CO2 (aq) during titration progress can be represented by the following equation (see Supporting Information for the derivation which is also

[CO2(aq)]tn − KHpCO2(g ) =

(1 − e−nkτ ) (1 − e−kτ )

ΔC e−k t (5)

where [CO2 (aq)]tn is the dissolved carbon dioxide molarity at time t following the addition of the nth aliquot of the acid. As shown in Figure 7, this model, though being able to predict the general shapes of the buffer capacity profiles to the point of being useful in terms of providing qualitative explanations, might be too simplistic to provide highly accurate quantitative predictions, and the development of a more nuanced mass transfer model is necessary for this purpose in the future. 5.2. Significance for in Vivo Predictive Drug Dissolution (iPD). With the relatively low buffer capacity of the gastrointestinal luminal fluid (when evaluated as a phasehomogeneous system, since when the measurements presented in literature were performed, no sparging was performed for the intestinal fluid samples) and small fluid volumes reported in the intestine,10,11 the assumption of the bulk pH being unaffected by G


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Figure 6. An example of a [CO2 (aq)] vs time profile obtained during the determination of CO2 mass transfer kinetics between the aqueous and gas phases as described in section 3.3.

the dissolution of a weakly acidic drug will not always hold, particularly in the case of high dose acidic drugs and if they are not very permeable. In addition, with the fasted intestinal pH values being around 6 proximally, 6.8−8 in the distal small intestine, and 6.5−7 in the colon,10,12,13 the described buffer capacity enhancement will be expected to be significant if the system is phase-heterogeneous with means for the rapid mass transfer of carbon dioxide, which appears to be the case in vivo in the human gastrointestinal tract including the colon. This actually seems to enable the intestinal fluids to buffer against pH shifts despite the relatively low buffer capacities measured in intestinal fluid ex vivo (which might also be further lowered when they are not titrated immediately after collection). In vivo, the CO2 mass transfer occurs through the permeation of dissolved CO2 across the intestinal epithelium into the bloodstream, ultimately carrying the CO2 to the lungs for exhalation. Data indicative of the intestinal permeability to CO2 are scarce. Endeward and Gros found that the CO2 permeability of colonic mucosa isolated from guinea pigs was close to 1 × 10−3 cm/s,3 which is a high value when compared to literature permeability values of different compounds.14 In addition, McIver et al.15 observed a rapid decrease in the volume of CO2 filled into a feline small intestinal loop (20 to 12 mL in 3 min). Therefore, the human gastrointestinal can be assumed to approximate a phase-heterogeneous system for carbon dioxide mass transfer. More investigations are needed to have a more reliable estimate for small intestinal CO2 permeability. As shown by Figure 5, the actual buffering action of the bicarbonate−CO2

system is a function of the acid input rate and the actual CO2 mass transfer rate (which in reality is finite). This means that when simulating the dissolution of a weak acid in a relatively low buffer capacity fluid (i.e., in human intestinal fluid), the following aspects need to be accounted for (in addition to using appropriate small volumes and media with the appropriate pH and low buffer capacity and other factors): 1. The bicarbonate secretion rate (basal pancreatic output rate around 0.5 mmol/h and basal mucosal duodenal output around 143 μmol h−1 cm−1).16,17 2. The generation of CO2 rate in the duodenum through proper simulation of bicarbonate secretion and acid input from the stomach (basal acid output in the stomach is around 3.3 mmol/h,18 while the initial gastric emptying half-life following administration of 240 mL of water seems to be around 13.5 min11). 3. The intestinal absorption of the dissolved drug. 4. The intestinal absorption of CO2 (by controlling the carbon dioxide mass transfer to maintain it at a physiologically relevant rate). When these aspects are properly accounted for, then the bulk pH during drug dissolution will be maintained at values that properly reflect the in vivo situation, and the in vitro dissolution will be more in vivo predictive. The first three aspects were taken, directly or indirectly, into account by various apparatus setups,19 but to date no systematic attempt at taking the fourth aspect (i.e., CO2 mass transfer kinetics) into account has been reported. H


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Figure 7. Predicted vs observed buffer capacity profiles for Setups A, B, and C.

dissolution volume used (900 mL) prevented the pH shifts from translating to pronounced differences between the dissolution profiles obtained using the different setups. However, it points to the need to consider the CO2 transport kinetics in experimental design. In the dynamic (also sparged) dissolution system used by Goyanes et al., no pH shifts were observed with the same products further supporting this notion.25 It is important for this property of the bicarbonate buffer to be taken into consideration while designing an in vivo Predictive Dissolution (iPD) setup. In order to obtain the correct degree of buffer capacity enhancement, the CO2 mass transfer rate in vitro should be reflective of the in vivo mass transfer when a bicarbonate buffer is used or that this effect is accounted for when determining the composition of a surrogate nonvolatile (e.g., phosphate) buffer. The different dependence of buffer capacity on pH, and its particular dependence on bicarbonate concentration (eq 2), also need to be considered. This makes a substitution of bicarbonate with another buffer dependent on pH. Given the variable pH in the intestine, it might not be difficult to match the in vivo bicarbonate buffer capacity over a large pH range with a surrogate nonvolatile buffer. In this regard, the bicarbonate-based dynamic dissolution testing devices, the use of which has been reported in several publications, can be useful.25−29 These devices attempt to mimic the pH gradient in the intestine through sparging the dissolution

The steep increase in the degree of this buffer capacity enhancement at pH values above the pKa of bicarbonate implies that the significance of this effect with regard to acidic drug dissolution becomes greater in the distal regions of the gastrointestinal tract. This is also likely of significance for enteric coated pharmaceutical products targeted to the lower gastrointestinal tract. A particular case worth investigating in this regard would be delayed release formulations of 5-aminosalicylic acid (like Asacol). In this case, high doses of 5-aminosalicylic acid (up to 1600 mg)20 are released from a tablet coated with a pH-dependent coat21 into the terminal ileum and the colon (where the fluid volumes are very small22), and additionally, in the colon, the permeability of the drug is low.23 This makes it highly likely for the described buffer capacity enhancement of the bicarbonate−CO2 system to play a significant role in driving the dissolution of 5-aminosalicylic acid. An indication toward this is found in the work of Fadda et al.,24 where, during the testing of delayed release 5-aminosalicylic acid tablets, a pH drop (from 7.4 to 7.0 with the 400 mg tablets and from 7.4 to 6.7 with the 800 mg tablets) was observed in a phase-homogeneous bicarbonate system (a setup with a paraffin oil layer at the surface which is assumed to limit the loss of CO2 from the aqueous phase, thus making the system behave as if it were a phase-homogeneous one), while no pH drop was observed in the setup sparged with 5% CO2. The large I


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compounds as well as gastric acid neutralization in the human intestine. We developed a mass transport analysis and validated it, showing that the system has a significantly enhanced buffer capacity in a physiologically important manner. Future work is planned involving investigating the actual hydration/dehydration kinetics, and interplay between acid addition rate and CO2 mass transfer rate is necessary to further characterize this bicarbonate−CO2 buffer system in vivo. Additional factors in vivo such as drug (acidic or basic) permeation, intestinal fluid bicarbonate molarity, and pCO2 as well as the fluid volumes present and the permeability of the intestinal epithelia to CO2 and the acidic or basic drug need to be taken into account to further characterize the bicarbonate−CO2 buffering effect on acidic and basic drug dissolution/precipitation.

medium with CO2 and another neutral (from the viewpoint of its acid−base properties) gas, which makes these buffer systems phase-heterogeneous ones, and therefore, provided that sparging setups that result in gas−liquid CO2 mass transfer kinetics that reflect the intestinal CO2 absorption rates are employed, are well-positioned to reflect this buffering capacityenhancing effect of CO2 mass transfer. An additional implication is related to our ability to design a surrogate buffer based on a nonbicarbonate species (e.g., phosphate). In the boundary diffusion layer around a dissolving ionizable solid, the interconversion between CO2 and H2CO3 does not reach equilibrium at steady state, and this results in the buffer capacity of bicarbonate within the intestinal pH range being lower than what would be expected from a buffer with a pKa of 6.04.1 This means that bicarbonate has a lowered buffer capacity in terms of promoting drug ionization and dissolution at the surface of a dissolving solid drug particle, but in bulk, by virtue of its phase-heterogeneous nature, it has an enhanced buffer capacity in terms of resisting bulk pH changes as a result of drug dissolution. This means that while designing a surrogate buffer might be feasible in the case that sink conditions are met (with the resulting little accumulation of dissolved drug making any changes in bulk pH too small to be of significant impact), it might be difficult in the case those sink conditions are not met. For in nonsink conditions, where significant shifts in bulk pH are expected, a typical pharmaceutical buffer like phosphate or acetate will, in contrast to bicarbonate, have similar buffering properties in both the bulk and the boundary diffusion layer, and therefore, matching its buffering action to that of a physiological bicarbonate buffer for buffering the bulk will result in a mismatch in terms of buffering the drug−buffer interface and vice versa. Therefore, developing a surrogate buffer might be more difficult in the case of in vivo predictive dissolution testing of ionizable BCS class III and IV compounds. 5.3. Buffer Capacity in the Base-Adding Direction. Since the derivation approach of eq 2 by Butler also applies the scenario of base addition,9 eq 1 applies also for the base-adding scenario. In this case, the phase-heterogeneous nature of the buffer would lead to enhanced buffer capacity due to replacement of the CO2 (aq) consumed by the base. In contrast to progressive acid addition which leads to a gradual drop in buffer capacity due to bicarbonate consumption, progressive addition of a base will lead to a gradual increase in buffer capacity owing to fixation of dissolved carbon dioxide as bicarbonate resulting in increased bicarbonate molarity. So asymmetric buffer capacity profiles similar to the one obtained for bicarbonate in Figure 3 will be obtained; as time progresses, buffer capcity will be increasing as the pH moves from the lefthand side to the right-hand side. This will also occur in the body, since CO2 is produced by cells everywhere allowing for compensation of the consumed dissolved CO2. This means that overall, human tissues seem to be more capable of resisting alkalinization than acidification, which might be connected to the fact that, in the case of acid−base homeostasis disturbances, pH values below 6.8 (0.6 units below normal) are considered fatal while in alkalosis a pH of 7.8 (0.4 units above normal) is considered fatal.30

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.molpharmaceut.8b00783.

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Detailed information regarding the mathematical derivation of eqs 1, 3, 4, and 5 in the manuscript (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Daniel P. McNamara: 0000-0001-5785-2405 Bart Hens: 0000-0002-4229-9843 Gordon L. Amidon: 0000-0003-0355-911X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by FDA Contract HHSF223201510157C. This article reflects the views of the authors and should not be construed to represent the FDA’s views or policies. We would also like to thank Dr. Abdul W. Basit (School of Pharmacy London) and Dr. Hamid Merchant (University of Huddersfield) for the helpful discussions. Bart Hens acknowledges the ‘Fonds voor Wetenschappelijk Onderzoek (FWO - 12R2119N.

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REFERENCES

(1) Krieg, B. J.; Taghavi, S. M.; Amidon, G. L.; Amidon, G. E. In vivo predictive dissolution: transport analysis of the CO2, bicarbonate in vivo buffer system. J. Pharm. Sci. 2014, 103, 3473−3490. (2) Krieg, B. J.; Taghavi, S. M.; Amidon, G. L.; Amidon, G. E. In vivo predictive dissolution: comparing the effect of bicarbonate and phosphate buffer on the dissolution of weak acids and weak bases. J. Pharm. Sci. 2015, 104, 2894−2904. (3) Endeward, V.; Gros, G. Low carbon dioxide permeability of the apical epithelial membrane of guinea-pig colon. J. Physiol. 2005, 567, 253−265. (4) Hulanicki, A. Reactions of acids and bases in analytical chemistry; E. Horwood: Chichester, West Sussex, U.K.; Halsted Press: New York, 1987. (5) Hens, B.; Tsume, Y.; Bermejo, M.; Paixao, P.; Koenigsnecht, M. J.; Baker, J. R.; Hasler, W. L.; Lionberger, R.; Fan, J.; Dickens, J.; Shedden, K.; Wen, B.; Wysocky, J.; Loebenberg, R.; Lee, A.; Frances, A.; Amidon, G. E.; Yu, A.; Benninghoff, G.; Salehi, N.; Talatoff, A.; Sun, D.; Amidon, G. L. Low Buffer Capacity and Alternating Motility along the Human Gastrointestinal Tract: Implications for in Vivo Dissolution and

6. SUMMARY The volatility of dissolved CO2 has received little attention in routine pharmaceutical laboratory practice despite it being a significant aspect of its interaction with weakly acidic and basic J


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Molecular Pharmaceutics Absorption of Ionizable Drugs. Mol. Pharmaceutics 2017, 14, 4281− 4294. (6) McNamara, D. P.; Whitney, K. M.; Goss, S. L. Use of a Physiologic Bicarbonate Buffer System for Dissolution Characterization of Ionizable Drugs. Pharm. Res. 2003, 20, 1641−1646. (7) Gilbert, D. L. Buffering of blood plasma. Yale J. Biol. Med. 1960, 32, 378−389. (8) Davenport, H. W. 1958. The ABC of Acid-Base Chemistry, 4th ed.; The University of Chicago Press: Chicago. (9) Butler, J. N. Carbon Dioxide Equilibria and Their Applications; Addison-Wesley Pub. Co.: Reading, MA, USA, 1982. (10) Mudie, D. M.; Amidon, G. L.; Amidon, G. E. Physiological parameters for oral delivery and in vitro testing. Mol. Pharmaceutics 2010, 7, 1388−1405. (11) Mudie, D. M.; Murray, K.; Hoad, C. L.; Pritchard, S. E.; Garnett, M. C.; Amidon, G. L.; Gowland, P. A.; Spiller, R. C.; Amidon, G. E.; Marciani, L. Quantification of gastrointestinal liquid volumes and distribution following a 240 mL dose of water in the fasted state. Mol. Pharmaceutics 2014, 11, 3039−3047. (12) Koziolek, M.; Grimm, M.; Becker, D.; Iordanov, V.; Zou, H.; Shimizu, J.; Wanke, C.; Garbacz, G.; Weistchies, W. Investigation of pH and temperature profilesin the GI tract of fasted human subjects using the Intellicap® system. J. Pharm. Sci. 2015, 104, 2855−2863. (13) Evans, D. F.; Pye, G.; Bramley, R.; Clark, A. G.; Dyson, T. J.; Hardcastle, J. D. Measurement of gastrointestinal pH profiles in normal ambulant human subjects. Gut 1988, 29, 1035−1041. (14) Lennernäs, H.; Nylander, S.; Ungell, A. L. Jejunal permeability: a comparison between the Ussing chamber technique and the single-pass perfusion in humans. Pharm. Res. 1997, 14, 667−671. (15) McIver, M. A.; Redfield, A. C.; Benedict, E. B. Gaseous exchange between the blood and the lumen of the stomach and intestines. Am. J. Physiol. 1926, 76, 92−111. (16) Isenberg, J. I.; Cano, R.; Bloom, S. R. Effect of graded amounts of acid instilled into the duodenum on pancreatic bicarbonate secretion and plasma secretin in duodenal ulcer patients and normal subjects. Gastroenterology 1977, 72, 6−8. (17) Isenberg, J. I.; Hogan, D. L.; Koss, M. A.; Selling, J. A. Human duodenal mucosal bicarbonate secretion: Evidence for basal secretion and stimulation by hydrochloric acid and a synthetic prostaglandin E1 analogue. Gastroenterology 1986, 91, 370−378. (18) Blair, J. A.; Feldman, M.; Barnett, C.; Walsh, J. H.; Richardson, C. T. Detailed comparison of basal and food-stimulated gastric acid secretion rates and serum gastrin concentrations in duodenal ulcer patients and normal subjects. J. Clin. Invest. 1987, 79, 582−587. (19) McAllister, M. Dynamic dissolution: A step closer to predictive dissolution testing? Mol. Pharmaceutics 2010, 7, 1374−1387. (20) Allergan Inc. https://www.asacolhd.com/. Accessed on 06.06.2017. (21) Al-Gousous, J.; Tsume, Y.; Fu, M.; Salem, I. I.; Langguth, P. Unpredictable performance of pH-dependent coatings accentuates the need for improved predictive in vitro systems. Mol. Pharmaceutics 2017, 14, 4209−4219. (22) Schiller, C.; Fröhlich, C. P.; Giessmann, T.; Siegmund, W.; Mönnikes, H.; Hosten, N.; Weitschies, W. Intestinal fluid volumes and transit of dosage forms as assessed by magnetic resonance imaging. Aliment. Pharmacol. Ther. 2005, 22, 971−979. (23) Andreas, C. J.; Ying-Chen, C.; Markopoulos, C.; Reppas, C.; Dressman, J. In vitro biorelevant models for evaluating modified release mesalamine products to forecast the effect of formulation and meal intake on drug release. Eur. J. Pharm. Biopharm. 2015, 97, 39−50. (24) Fadda, H. M.; Merchant, H. A.; Arafat, B. T.; Basit, A. W. Physiological bicarbonate buffers: stabilization and use as dissolution media for modified release systems. Int. J. Pharm. 2009, 382, 56−60. (25) Goyanes, A.; Hatton, G. B.; Merchant, H. A.; Basit, A. W. Gastrointestinal release behaviour of modified-release drug products: dynamic dissolution testing of mesalazine formulations. Int. J. Pharm. 2015, 484, 103−108. (26) Merchant, H. A.; Goyanes, A.; Parashar, N.; Basit, A. W. Predicting the gastrointestinal behaviour of modified-release products:

utility of a novel dynamic dissolution test apparatus involving the use of bicarbonate buffers. Int. J. Pharm. 2014, 475, 585−591. (27) Wulff, R.; Rappen, G. M.; Koziolek, M.; Garbacz, G.; Leopold, C. S. Controlled release of acidic drugs in compendial and physiological hydrogen carbonate buffer from polymer blend-coated oral solid dosage forms. Eur. J. Pharm. Sci. 2015, 77, 246−253. (28) Fina, F.; Goyanes, A.; Gaisford, S.; Basit, A. W. Selective laser sintering (SLS) 3D printing of medicines. Int. J. Pharm. 2017, 529, 285−293. (29) Garbacz, G.; Kołodziej, B.; Koziolek, M.; Weitschies, W.; Klein, S. A dynamic system for the simulation of fasting luminal pH-gradients using hydrogen carbonate buffers for dissolution testing of ionisable compounds. Eur. J. Pharm. Sci. 2014, 51, 224−231. (30) Hale, A.; Hovey, M. J. Fluid, Electrolyte, and Acid-Base Imbalances: Content Review Plus Practice Questions; F. A. Davis Company: Philadelphia, PA, USA, 2014.

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Mass Transport Analysis of the Enhanced Buffer Capacity of the

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