Caffeine is a cardinal nervous system ( CNS ) stimulation, known medically as trimethylxanthine with a chemical expression of C8H10N4O2 [ 1 ] . It is soluble 1 in 46 of H2O, has a dissociation changeless pKa of 0.6 [ 2 ] , and a log P value of -0.07 [ 1 ] . From the construction of caffeine ( fig. 1 ) , three amide groups and two aromatic aminoalkane groups could be observed. Although the amide groups of caffeine are impersonal, the presence of the aromatic aminoalkanes groups indicates that caffeine is a drug with weak basic belongingss.

Caffeine is found of course in java, tea and cocoa, and sometimes is used as an added energy encouragement in energy drinks or soft drinks, therefore doing it the universe ‘s most widely consumed psychotropic substance. Unlike many other psychotropic substances, caffeine is legal and has been listed as a “ multiple intent by and large recognized as safe nutrient substance ” by the Food and Drug Administration [ 3 ] . It is used both medically and recreationally to cut down physical weariness, ward off sleepiness and reconstruct mental watchfulness. Besides, it is sometimes used to increase urine production due to its mild diuretic belongingss [ 4 ] . Caffeine is besides included in some common nonprescription medicines such as acetylsalicylic acid and appetite suppressants, for the intent of heightening the consequence of the primary active ingredient or cut downing the drowsiness side consequence [ 5 ] . Apart from the unwritten preparation, caffeine is besides available in other assorted preparations such as intramuscular injections and suppositories. The suppositories with the ingredients of ergotamine tartrate and caffeine are used to relief migraine [ 6 ] while others with the ingredients of chlorobutanol and caffeine are used to handle hyperemesis [ 7 ] .

1.2. Mechanism of Action

Caffeine Acts of the Apostless as a nonselective adversary of adenosine receptors in the encephalon [ 5 ] , therefore it readily crosses the blood-brain barrier to exhibit its manner of action. Once in the encephalon, caffeine Acts of the Apostless as a competitory inhibitor and binds to the adenosine receptors on the surface of cells as it is structurally similar to adenosine ( fig. 2 ) .

Figure 2, constructions of caffeine and adenosine.

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The binding of adenosine to the receptor usually causes sleepiness by decelerating down nerve cell activity. However, the binding of caffeine to the adenosine receptor is an antagonist mechanism of action. Alternatively of decelerating down the cell ‘s activity, caffeine causes the nervus cells to rush up, hence holding the consequence of cut downing sleepiness or increasing watchfulness. Besides, caffeine besides causes the bottleneck of encephalon ‘s blood vass, helped in alleviating concern or megrim [ 4 ] . Besides, due to the increased nerve cell fire in the encephalon, the pituitary secretory organ so releases endocrines that stimulate the adrenal secretory organs to release adrenaline [ 4 ] . Adrenaline is the “ battle or flight ” endocrine which helps the organic structure to set to sudden emphasis. It increases the rate of pulse, raises the blood force per unit area, every bit good as velocities up the transition of animal starch into glucose to supply excess energy to the musculuss. This is farther explained why caffeine is widely used to ease fatigue and restore watchfulness.

1.3. Pharmacokineticss

In the find of new drugs, the ability of the drug to divider and arrive at its site of action is of great importance. Many drug campaigner molecules are discarded due to hapless pharmacokinetic belongingss affecting physicochemical parametric quantities of soaking up, distribution, metamorphosis and riddance ( ADME ) . Avdeef et al [ 2 ] stated that hapless solubility and permeableness history for many pharmacokinetic failures. The pharmacokinetics of the drugs are dependent on their ability to fade out in the organic structure and so divider across assorted cellular membranes into the blood and later into mark tissues, by different conveyance mechanisms, to exhibit their manner of actions. However, the ability of drugs to divider across cellular membranes depends on a assortment of factors such as the drug ‘s molecular size, grade of ionization, lipophilicity and etc. Therefore, the survey and word picture of the factors regulating the pharmacokinetics of caffeine is really of import as they play an of import function in ADME.

Pharmacokineticss belongingss of caffeine show that the soaking up of caffeine chiefly takes topographic point in the tummy and little bowel in the organic structure. There is a survey demoing that caffeine from java or other drinks is absorbed within 45 proceedingss of consumption [ 8 ] . The same survey besides stated that caffeine can be distributed throughout all the tissues in the organic structure [ 8 ] . However, the metamorphosis of caffeine takes topographic point in the liver and the three metabolites produced are Paraxanthine, Theobromine and Theophylline [ 9 ] . The riddance of caffeine is a first-order reaction [ 10 ] .

1.4. Breakdown of Caffeine

Partitioning of drugs is defined as the motion or redistribution of drug molecules from one liquid to another. When an surplus of drug compounds is added to a system of two non-miscible liquids which approximates the cellular membranes, it will partition itself between the two liquids until each becomes saturated. However, if the sum of drug compounds added is non sufficient to saturate the liquids, it will administer between the two non-miscible liquids harmonizing to a definite concentration ratio. Polar drugs normally favour the aqueous stage while non-polar drugs normally favour the organic stage. The breakdown of a drug is affected by assorted molecular belongingss, for illustration molecular size, lipophilicity and charge ( grade of ionization ) of the drug [ 17 ] .

By and large, the breakdown of a drug across the membrane additions with increasing lipophilicity [ 17 ] , therefore bespeaking the importance of the lipophilicity of a drug. The importance of the lipophilicity is farther supported by the transition of drug into the cardinal nervous system ( CNS ) by traversing the blood-brain barrier ( BBB ) [ 17 ] . As BBB is composed of many tight junctions in between endothelial cells, the sum of drug that is able to partition across BBB is reduced to a greater extent compared to the sum of drug that is able to partition across GIT. As a consequence, the lipophilicity of a drug plays a major function in the transition of drug across BBB. Caffeine is a CNS stimulation, so it must be able to traverse the BBB to exercise its stimulating consequence in the encephalon. Therefore, the lipophilicity of caffeine should be investigated.

The lipophilicity of a drug is normally determined by the Partition Coefficient ( P ) , which is defined as the ratio of concentrations of un-ionised compound between the two stages. Therefore, the lipophilicity of caffeine can be determined utilizing the divider coefficient equation as follow:

Where Corg is the concentration of the drug in organic bed while Caq is the concentration of the drug in aqueous bed

As the divider coefficient of a drug provides a thermodynamic step of its hydrophilicity-lipophilicity balance, it is widely used to foretell the soaking up, distribution and riddance of drugs within the organic structure. The divider coefficient of un-ionised drug is called the True Partition Coefficient ( PTRUE ) . The drug is hydrophobic ( or lipophilic ) when PTRUE value is greater than one due to the higher concentration of drug in the organic stage compared to the aqueous stage. However, the drug is hydrophilic ( or lipophobic ) when PTRUE value is less than one as a consequence of a lower concentration in the organic stage compared to the aqueous stage.

Alternatively, Log P can be used to find caffeine ‘s lipophilicity. It is defined as the logarithm of the ratio of the concentrations of the un-ionised compound in the two non-miscible stages, which can be written as:

When log P value is greater than zero, the solubility of the drug compound in the organic stage is greater than in the aqueous stage. When log P value peers to zero, the drug compound has an equal solubility in organic and aqueous stages. When log P value is smaller than zero, the solubility of the drug compound in organic stage is less than in aqueous stage. As mentioned before, caffeine has a log P value of -0.07, therefore bespeaking that caffeine is hydrophilic and its solubility in the aqueous stage is greater than in the organic stage.

Apart from that, the lipophilicity of a drug can besides be determined by Hansch-Fujita parametric quantities, which help in placing the functional groups that contribute to the lipophilicity of the drug. A predomination of hydrophobic groups ( +Iˆ ) will take to an addition in divider coefficient, therefore doing the drug more lipid soluble and able to go through through the lipid membranes more easy. However, a predomination of hydrophilic groups ( -Iˆ ) will take to a lessening in divider coefficient, doing the drug less lipid soluble and less likely to perforate the lipid membranes.

Therefore, the lipophilicity of caffeine could besides be examined by the usage of Hansch-Fujita parametric quantities. As can be seen from the construction ( fig.1 ) , caffeine has few hydrophilic parts represented by aminoalkane and amide groups. These hydrophilic groups would cut down the soaking up of caffeine, therefore taking to a decrease of pharmacological consequence exerted by caffeine. However, caffeine besides contains few hydrophobic parts, which are represented by aromatic and aliphatic hydrocarbons. These hydrophobic groups would increase the lipophilicity of caffeine, therefore leting it to be absorbed into the organic structure, particularly across the BBB into the encephalon and exercise its action.

However, if the drug is a weak acid or a weak base so ionization will significantly change the breakdown of drug as merely the uncharged or un-ionised molecules will partition into the hydrophobic cellular membrane efficaciously by inactive diffusion. Given that caffeine is a weak base, log P is hence non an appropriate forecaster to find the lipophilicity for caffeine due to its ionisable belongingss and log P merely describes the divider coefficient of impersonal or un-ionised drug molecules.

The right forecaster for ionisable drugs which can be used is the Distribution Coefficient ( Log D ) . It is defined as the ratio of the amount of the concentrations of all ionized and un-ionised signifiers of the drug in both stages, which can be written as:

Without taking into history the ionization of the drug, log D of the drug at a given pH can besides be estimated by cognizing its log P and pKa values [ 16 ] . Caffeine is a basic drug with a log P value of -0.07 and a pKa value of 0.6, therefore log D of caffeine at a given pH can be calculated, utilizing the equation as follow:

Alternatively, the Apparent Partition Coefficient ( PAPPARENT ) can be used to foretell caffeine ‘s behavior at changing pH environments in the organic structure, utilizing the equation as defined below:

PAPPARENT = PTRUE A- funionised

Where funionised is the fraction of drug which is un-ionised at certain pH

As mentioned antecedently, the grade of ionization of a drug is another of import factor impacting the breakdown belongingss of a drug. Therefore, the grade of ionization is besides a utile parametric quantity for foretelling the solubility and the soaking up of a drug. The grade of ionization is dependent on both the pH of the solution and the pKa of the drug itself. For that ground, both pH and pKa have to be taken into consideration in finding the breakdown belongingss of a drug. The pKa ( acerb dissociation invariable ) of a drug is a physicochemical parametric quantity, widely used to bespeak the grade of ionization of a drug with regard to the pH of the environment, therefore foretelling the sum of drug soaking up at a peculiar site within the organic structure. Harmonizing to the pH-partition hypothesis, merely the un-ionised non-polar drug permeates the cellular membranes [ 17 ] , indicated that breakdown of a drug mostly depends on the pH of the environments due to its consequence on the grade of ionization of the drug. In the organic structure, the GI piece of land ( GIT ) exhibits a important pH gradient, from every bit low as 1.5 in the tummy to every bit high as 8.0 in the little bowel, therefore the pH-partition hypothesis predicts that the soaking up of ionisable drugs might change across the GIT due to the difference in grade of ionization at different pH [ 2 ] .

As caffeine is a weak base, it will go ionized in solution and the grade of ionization will so change its breakdown as merely the un-ionised signifiers of molecules will perforate the cellular membranes efficaciously by inactive diffusion, as antecedently described. Knowing the pKa value of caffeine allows us to cipher its grade of ionization at different sites in the organic structure ( fig. 3 ) , therefore bespeaking the site that caffeine is most likely to be absorbed. The grade of ionization of caffeine at certain pH can be predicted utilizing the equation as follow:

Sites in the organic structure

pH

Percentage of ionization

Stomach

1.5

11.2 %

Duodenum

5.5

1.3 ten 10-3 %

Ileum

8.0

4.0 ten 10-6 %

Blood

7.4

1.6 ten 10-5 %

Figure 3, table demoing the per centum of ionization of caffeine at different sites in the organic structure

As can be observed from the tabular array above ( fig. 3 ) , the per centum of ionization of caffeine is comparatively little at all pH conditions in the organic structure, indicated that it is mostly un-ionised in the organic structure, therefore demoing that caffeine is readily absorbed every bit shortly as it reaches the tummy.

Although lipophilicity and grade of ionization are the cardinal factors impacting the breakdown of caffeine across the cellular membranes, belongingss and features of the membrane are besides really of import [ 17 ] . They are many different types of cellular membranes nowadays in the organic structure, for illustration gastro-intestinal piece of land ( GIT ) and blood-brain barriers ( BBB ) . Besides, they are composed of different thicknesses or types of constituents, thereby act uponing the interactions between drug molecules and membranes every bit good as impacting the breakdown of a drug across the membranes. As a consequence, apart from look intoing the breakdown of caffeine between the organic and aqueous stages, it is besides of import to analyze the interactions between caffeine and membrane constituents.

1.5. Cellular Membranes

It is of import to cognize that in the organic structure, there are many barriers which a drug has to traverse before it becomes effectual, for illustration, GI piece of land ( GIT ) and blood-brain barriers ( BBB ) . All the life cells in the organic structure are surrounded by cell membrane, dividing the interior portion from the outside environment, every bit good as commanding the motion of substances in and out of cells [ 11 ] . Cellular membranes are composed chiefly of a bed of amphipathic phospholipids, with the hydrocarbon tails being hydrophobic whereas the polar caput groups being hydrophilic. When dispersed in aqueous solution, the phospholipids are arranged to organize a lipid bilayer so that the H2O “ hating ” hydrophobic dress suits are shielded by the H2O “ loving ” hydrophilic caput groups from the environing aqueous environment ( fig. 4 ) .

Figure 4, construction of cellular membrane

Phospholipids are the major category of lipoids in cell membranes, composed of many different constituents such as phosphatidylcholine ( Personal computer ) , phosphatidylserine ( PS ) , phosphatidylethanolamine ( PE ) and etc. Personal computer is normally the most abundant phospholipid found in mammalian cells, peculiarly in the outer cusp of the plasma membranes [ 12 ] , whereas PS is merely found less than 10 % of the entire phospholipids, chiefly in medulla from encephalon tissue [ 13 ] . Personal computer is a zwitterionic ( impersonal ) phospholipid [ 12 ] while PS is an anionic ( acidic ) phospholipid [ 13 ] at physiological pH ( fig. 5 and 6 ) . These constituents are varied in construction, grade of impregnation and charge province, therefore impacting the ability of drugs to perforate the membranes.

Figure 5, construction of phosphatidylcholine ( Personal computer )

Figure 6, construction of phosphatidylserine ( PS )

As a consequence, cellular membranes are the barriers that drugs ( caffeine ) have to traverse to make their biological marks and exhibit their actions. In order for caffeine to be absorbed, it must go through from the aqueous solution of the extracellular fluid ( outside of cell ) to the aqueous solution of the intracellular fluid ( inside of cell ) by traversing the lipid membrane. For efficient conveyance, caffeine must be sufficiently soluble in both aqueous and the lipid stages to partition to its site of action ( i.e. caffeine must be hydrophobic plenty to partition into the lipid membrane, but non excessively hydrophobic until it is unable to partition out into mark tissues ) [ 14 ] . Besides, the features of membranes should be taken into consideration as good while analyzing the partitioning behavior of caffeine as they are composed of different constituents, as antecedently described. Therefore, the probes of belongingss and features of both caffeine and membrane are of great importance as they contribute assorted factors to impact the soaking up and breakdown of caffeine in the organic structure.

1.6. In vitro Models

In the development of new drug compounds, the anticipation of in vivo drug soaking up is a large concern. Therefore, assorted in vitro theoretical accounts were developed to foretell the soaking up of drugs from the partitioning behavior in biphasic systems [ 15 ] . These theoretical accounts normally consist of two non-miscible stages, one of which is composed of H2O or aqueous buffer and the other is composed of organic dissolver. The normally used organic dissolvers are n-octanol, cyclohexane, trichloromethane or heptane. The motion ( partitioning ) of drugs from one stage to another is so assessed. There is a assortment of systems being developed to analyze the lipophilicity and breakdown of drugs, for illustration octanol-water system, cyclohexane-water system, liposomal systems, chromatography systems and etc [ 15 ] .

For many decennaries, the n-octanol-water theoretical account is the method of pick in foretelling drug soaking up [ 15 ] . The usage of water-saturated octanol or “ wet octanol ” as the organic stage is thought to be the best as the water-saturated octanol shows a more complex construction compared to the pure octanol, giving it a more similar construction to that of the phospholipid bilayer [ 2 ] . About 25 mol % of H2O is thought to fade out in the water-saturated octanol, organizing H2O bunchs that are surrounded by octanol [ 2 ] ( fig. 7 ) . The polar hydroxyl groups of octanol molecules are hydrogen-bonded to the H2O molecules while the aliphatic dress suits of octanol molecules form a hydrophobic part similar to the hydrocarbon nucleus of phospholipid bilayer [ 2 ] .

Figure 7, construction of water-saturated octanol derived from a low-angle X-ray diffraction survey [ 2 ]

In add-on, the H adhering features possessed by the water-saturated octanol are thought to be similar to those of cellular membranes, therefore proposing that the distribution of drugs into the octanol ( hydrophobic organic stage ) simulates, to a certain extent, their ability to spread passively across cellular membranes.

However, there are few issues being raised with the usage of octanol-water theoretical account. The octanol-water theoretical account approximates the conveyance of drugs through the cellular membranes taking into history merely the inactive transcellular diffusion mechanisms. In world, the transition of drugs across the cellular membranes might be occurred through different conveyance mechanisms, for illustration channel-mediated or carrier-mediated inactive conveyance and active conveyance ( fig. 8 ) . The inactive conveyance involves the motion of drug molecules down a concentration gradient while the active conveyance involves the motion of drug molecules against its concentration gradient with the usage of energy.

Figure 8, different types of conveyance mechanisms.

Besides, the construction of cellular membrane in the organic structure is far more complex compared to the simple two-phase octanol-water theoretical account as in add-on to phospholipids, cellular membrane besides contains cholesterin, glycolipids and proteins which might impact the breakdown of drugs.

Despite these ascertained differences, the octanol-water theoretical account is still being used in this undertaking to find the partitioning behavior of caffeine due to its simpleness and belongingss that simulates the cellular membranes in the organic structure.

PAMPA involved utilizing a membrane filter which contains different phospholipids to foretell in vivo activity while Caco-2 cells mimic the enteric epithelial cells to find drug transportation/ divider across enteric cells. [ 18 ] [ 19 ]

18. K. Sugano, Artificial Membrane Technologies to Assess Transfer and Permeation of Drugs in Drug Discovery, In: John B. Taylor and David J. Triggle, Editor ( s ) -in-Chief, Comprehensive Medicinal Chemistry II, Elsevier, Oxford, 2007, Pages 453-487

19. P. Artursson et Al, Passive Permeability and Active Transport Models for the Prediction of Oral Absorption, In: John B. Taylor and David J. Triggle, Editor ( s ) -in-Chief, Comprehensive Medicinal Chemistry II, Elsevier, Oxford, 2007, Pages 259-278

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