Innovations In Prodrug Approaches For Enhanced Bioavailability

The idea of prodrugs, biologically inactive analogs of active drugs that can be converted to give the active parent drug in vivo, has developed through a complex strategy of enhancing solubility of drugs to an elaborate pharmaceutical solution to several drug development and delivery issues. With the discovery of how about 10 per cent of the drugs approved are prodrugs, pharmaceutical scientists have been adopting this approach as a potent means of overcoming the pharmacodynamics and pharmacokinetic drawbacks.[1]

Bioavailability, defined as the fraction of an administered dose that reaches systemic circulation in unchanged form, represents a critical determinant of drug efficacy and safety. For orally administered drugs, bioavailability is influenced by multiple factors including solubility, permeability across biological membranes, metabolic stability, and susceptibility to efflux transport. The Biopharmaceutics Classification System (BCS) categorizes drugs based on solubility and permeability characteristics, with BCS Class II (low solubility, high permeability) and Class IV (low solubility, low permeability) drugs presenting particular challenges for achieving adequate bioavailability.[2]

Formulation techniques that have been used to improve bioavailability in traditional methods have been particle size reduction, salt formation, and excipient complexation. Although these techniques are still useful, they can frequently be inadequate with highly lipophilic compounds, drugs with a large extent of first-pass metabolism, or molecules that are efflux transporter substrates. The prodrug solutions are complementary or alternative solutions in that they essentially modify the properties of the drug at the absorption stage but retain its therapeutic potential on activation.[3]

The recent developments in the knowledge of the absorption mechanisms, the biology of the transporters and the metabolic pathways have allowed more rational design of prodrugs. This has resulted in carrier-targeted prodrugs that utilise endogenous uptake capabilities and was made possible by the identification of certain transporters that are expressed in intestinal epithelium. On the same note, the development of the knowledge of the expression patterns of this enzyme in various tissues has enabled the development of tissue-specific prodrugs, which are selectively activated at the target location.[4]

The variety of promoieties that have been designed to be used in prodrug construction has increased with innovations in chemical synthesis and analysis techniques. In addition to straightforward ester and amide linkages, recent prodrug approaches incorporate sophisticated chemical designs such as cyclic designs, self-immolative linkers and stimuli-responsive groups that allow the activation kinetics and location to be precisely controlled. Combination of computational modeling with experimental validation has enhanced optimization of prodrugs, whereby, prediction of physicochemical properties, metabolic stability and transporter interactions can be made.[5]

This work will discuss the current developments in prodrug strategies to improve bioavailability, classified by mechanism and use. Our areas of interest include those strategies which have shown clinical or preclinical efficacy, new technologies with translational potential and underlying principles that inform contemporary prodrug design. The aim is to offer a complete picture of the latest prodrug strategies and future outlook of this dynamic area to pharmaceutical scientists and medicinal chemists.[6]

1. Classification of prodrug:

1.1 Carrier linked prodrug

Prodrugs formed as carrier-linked probothermal include chemically binding an active drug molecule to a promotory (carrier group) by a breakable group. This conjugation is supposed to be stable enough when in a physiological environment, such that, once administered, it may be broken by an enzyme or a chemical into liberating the active parent drug in vivo.[7]

This method is especially practical when dealing with drugs the application of which as a therapeutic agent is hampered by undesirable physicochemical characteristics of low solubility, low permeability or lack of stability. These properties can be altered temporarily, though by conjugating the drug with an appropriate carrier group, without changing the underlying pharmacological activity of parent compound. After cleavage of the prodrug in the body, the active drug is again synthesized, and it produces the desired effect on the body.[8]

A perfect carrier group needs to be non-toxic and biologically inactive and readily removed out of the body once the drug is released. The choice of the promoiety is very important as it dictates the stability, rate of activation and the overall activity of the prodrug.[9]

The dipivalyl ester of epinephrine using is a classic example of this type of drugs being used, as it enhances corneal tissue penetration and minimizes loss of bioactivity of the medication due to rapid metabolism, which increases systemic side effects.[10]

Carrier-linked are classified into based on the nature and the activation mechanism.

1. Cascade-Latentiated Prodrugs: The prodrugs then go through series of enzymatic changes with each conversion, resulting in the first conversion stage which gives an intermediate prodrug, which is further broken down to release the active drug.[11]
2. Double Prodrugs : To be activated, double prodrugs need two-step activation. The compound given is first changed into some intermediate form that is metabolized into giving the active drug.[12]
3. Macromolecular Prodrugs: Here, the drug is coupled to huge carrier molecules that enhance drug stability, delays the rate of circulation and allows controlled release of drug.[13]
4. Site-Specific Prodrugs: They are used to transport a drug to a certain tissue or organ. Activation is selective to the target site owing to the existence of particular enzymes or environmental conditions leading to a high efficacy and low toxicity in therapy. This would be most advantageous where there is cancer and localized infections.[14]

1.2 Bioprecursor Prodrug

Bio precursor prodrugs are such substances which lack a carrier (promoiety) group. Rather, they are created by a structural alteration of the active drug itself, and are transformed into the active form via a metabolic/chemical change in the body.[15]

Bioactivation of bioprecursor prodrugs does not happen through the cleavage confluence of a linkage as in the case of carrier-linked prodrugs. Instead, the molecule already contains an inactive functional group (or inactive form of the active group), so-called latent functional group, which is changed to the pharmacologically active species, through biochemical reactions.[16]

Such changes often entail:

•           Oxidation

•           Reduction

•           Phosphorylation

Oxidative Bioactivation

Oxidative reactions are an important route of activation of bioprecursor prodrugs. Making the inactive compound active is done, in this process, by metabolic enzymes (mainly in the liver) which oxidize the inactive compound to its active counterpart.[17]

An example in point is the well-known nabumetone, a nonsteroidal anti-inflammatory drug (NSAID). Nabumetone, unlike most of the NSAIDs, has no free carboxylic acid group that is usually the cause of gastric irritation. This leads to less pain in the stomach. It is oxidatively metabolized and on being absorbed in the intestine, it will be converted to the active acidic metabolite, which will give it its anti-inflammatory effects.[18]

Reductive Bioactivation

Reductive activation is not very prevalent as compared to the oxidative processes, there are only a few reducing cells in the body. It is critical in the design of the anticancer drugs.[19]

Mitomycin C is an instance of an antineoplastic agent which is applied in the treatment of cancer-like diseases, lung cancers and bladder cancer. It has a quinone moiety which is reduced by the enzymes to hydroquinone. Such a change in the electronic state of the molecule allows production of highly reactive intermediates that can alkylate DNA thereby causing cytotoxic activity on the cancer cell.[20]

1.3 Mutual Prodrug

Mutual prodrugs are systems that consist of two active drugs that are covalently bonded together where one drug acts as a promoiety to the other. When administered the bond between the two is broken upon physiological conditions releasing both the active drugs.[21]

This method can be conceptually likened to the rational drug design, in which drug compounds get altered to improve treatment effectiveness. Mutual prodrug design involves the selection of the components based on their complementary or synergistic mechanisms of action and delivery of two active agents at one site.[22]

The carrier drug can have a variety of applications. It may:

•           Have the same therapeutic effects and result in synergies.

•           Offer a different pharmacological action that is not available in parent drug.

•           Make it possible to deliver to a target organ or tissue site-specifically.

•           Aid in decreasing the undesirable effects of the parent drug.

Mutual prodrugs are of great help especially where combination therapy is a must since they are used to make sure that the two drugs are administered simultaneously in a controlled and effective form. It may lead to better therapeutic effect, enhanced drug delivery and decreasing toxicity.[23]

1.4 Polymeric Prodrug

Polymeric prodrug are systems where an active drug is covalently attached to a polymeric carrier, to create a macromolecular conjugate. Systems typically have been made to be biocompatible that is, they are non-toxic, non-immunogenic and safe therapeutically.[24]

The development of polymer-drug conjugates varies on a number of factors, such as chemical structure, molecular weight, steric effects and reactivity of the drug and the polymer. To be successfully conjugated, both of the components should have an appropriate functional group like -COOH, -OH, -NH 2 or -SH. Nevertheless, the existence of several reactive groups may complicate the synthesis and should be under control.[25]

The polymer carriers can be either powered or non-powered and depending on the mode of choice, the drug release profile and the biological behaviors may vary. The medication may be coupled to the polymer backbone or with an intermediate (so-called spacer) group which may be significant in determining how quickly and at what point the drug dissolves through enzymatic or hydrolytic variations.[26]

FUNDAMENTAL PRINCIPLES OF PRODRUG DESIGN

1. Bioavailability Barriers and Prodrug Solutions:

Figure 2. Major physiological barriers affecting oral drug bioavailability

The limited dissolution due to poor aqueous solubility of Class II and IV drugs can be a limiting factor to absorption because it forms an aqueous bottleneck. Introduction of ionizable groups, hydrophilic groups or structural groups can increase solubility via prodrug methods. Phosphate prodrug esters, such as allow greater water dissolution of the negatively charged phosphate atom in physiological pH and retain sufficient lipophilicity to enter membranes on dephosphorylation.[28]

Limitations in membrane permeability are based on the molecular attributes of size, charge and hydrogen bonding ability. A guideline of oral bioavailability is set by Lipinski (The Rule of Five), though in many essential cases of drug targets, molecules contravening such rules are necessary. Prodrug approaches can temporarily conceal polar functional groups, lower molecular weight by liberating promoieties or can use transporter-mediated uptake to circumvent permeability obstructions.[29]

Bioavailability of oxidable, conjugatable, or hydrolysable drugs can be significantly decreased by first-pass metabolism by intestinal mucosa and liver. Resistant prodrug designs, such as designs which resist premature metabolism or are saturation of a metabolic route of action, can enhance the proportion of drug in the systemic circulation. Also, prodrugs which can be activated by particular target tissue-enriched enzymes can avoid hepatic metabolism and specifically deliver to tissues.[30]

2. Chemical Strategies and Linkage Chemistry:

The promoiety-drug conjugation is another important design factor that defines stability, activation kinetic and bioavailability amplification. Ester bonds are the most prevalent since they can be synthesized and are hydrolyzed with certainty by widespread esterases. The stability of esters, however, can be significantly changed depending on steric and electronic factors and the activation rate can be tuned by several fold between minutes to hours.[31]

Substitutions with prodrugs have differing stability profiles and activation mechanisms, i.e., carbamate and carbonate prodrugs. Simple ester In comparison to simple derivatives, carbamate linkages tend to be more stable, yet susceptible to enzymatic degradation. Phenolic drugs have been successfully used with carbonate prodrugs, which offer increased stability in acidic environments, as well as give esterases and carbonic anhydrases an easy way in.[32]

Phosphate ester prodrugs are also a significant type especially with hydroxyl groups in the drugs. The phosphate group offers significant improvement in solubility but can be readily degraded by the intestinal-expressed alkaline phosphatases. An example of this approach is the phosphate ester prodrug of the HIV protease inhibitor amprenavir, called fosamprenavir, which exhibits better the bioavailability of the parent drug despite permeability similarity by 1.5-folds.[33]

Such complex chemical architectures as self-immolative linkers provide multi-step activation cascades. These systems include a spacer group between the drug and promoiety that spontaneously cleaves after the initial cleavage by an enzyme to release the parent drug by a cyclization or elimination reaction. This method has proven useful especially when it is chemically difficult to directly label the drug with the promoiety or controlled release kinetics are needed.[34]

3. Activation Mechanisms:

Activation Prodrug activation may be enzymatic or non-enzymatic, with each having its own benefits to the particular use. Tissue-targeted drug delivery may use enzymatic activation in the selectivity of enzyme expression patterns. Activating enzymes are common in the form of esterases, phosphatases, peptidases, and oxidoreductases and have distinct substrate specificities as well as tissue distribution.[35]

The most commonly used activating enzymes are carboxylesterases, which are highly expressed in liver and intestinal mucosa. Carboxylesterase 1 (hCE1) and carboxylesterase 2 (hCE2) have different substrate selectivity, as well as different expression patterns in tissues, and thus can be used to design prodrugs with selective activation profiles. Knowledge about speciation of carboxylesterase in expression and activity is important to overcome interdisciplinarity of preclinical data to clinical practice.[36]

The enzymes of cytochrome P450 can be utilized in the drug clearance and further metabolism but can be used to help activate prodrug by oxidation processes. This method is especially pertinent when it comes to the tissue-specific delivery because various P450 isoforms do not demonstrate the same expression patterns. Prodrugs activated by CYP3A4 such as the above can be preferentially activated in the intestinal mucosa and liver, where this enzyme is strongly expressed.[37]

In non-enzymatic activations, pH-dependent hydrolysis, endogenous reducing agents like glutathione, and photochemical cleavages are the methods of activation. pH-sensitive prodrugs are drugs that take advantage of variations in pH between different biological compartments to allow the drug to be delivered directly to the target. As an illustration, the Schiff base linkages in the body remain intact at neutral pH and readily dissolve in acid to enable the drug release in the stomach or acidic tumor micro-systems.[38]

CARRIER-TARGETED PRODRUG APPROACHES

1. Nutrient Transporter-Targeted Prodrugs:

The exploitation of endogenous nutrient transporters is one of the most effective methods of increasing oral bioavailability. The small intestines have many amino acids, peptides, vitamins, organic nucleoside and organic ions expressed transporters that allow the efficient intake of essential nutrients. Imitation of these natural substrates as prodrugs will significantly enhance the absorption of poorly permeable drugs.[39]

Prodrug Amino acid transporter has been especially successful. Amino acid transporter systems are represented by ATB0,+ and LAT1 that transports various types of amino acids with different efficiency. The conjugation of drugs to amino acids like valine, leucine or phenylalanine can gain recognition by these transporters and significantly increase cellular uptake. An example of this strategy is valacyclovir, the valine prodrug of acyclovir, which has 3-5 fold greater bioavailability than acyclovir because of its effective absorption by the peptide transporter PEPT1.[40]

The applicant product of SLC15A1 mimics the peptide transporter PEPT1 (SLC15A1), which is expressed largely in the apical membrane of intestinal enterocytes and takes di- and tripeptide as substrates. This transporter has been widely used in the design of prodrugs and there have been many successful cases in this category including prodrugs of 2-Lactam antibiotics that resemble dipeptide structures. PEPT1 recognition has the important structural conditions of having a free carboxyl group and amino group in the alpha position and proper distance between the functional groups.[41]

Figure 3. Transporter-mediated absorption of prodrugs

Another interesting target, however, to prodrug design can be monocarboxylases transporters (MCTs), especially when the drug to be conjugated is a short chain fatty acid drug. MCT1, which is found in intestinal epithelium, brain endothelium among other tissues, carries lactate, pyruvate, and other monocarboxylates. This transporter can be utilized by the use of enester prodrug conjugates to propionic or butyric acid and facilitate absorption and (probably) the delivery into the brain.[42]

2. Vitamin and Nucleoside Transporter-Mediated Delivery:

Vitamin transporters are an excellent transporter system that can be used to deliver prodrug. Folates are taken into cells by the proton-coupled folate transporter (PCFT) and reduced folate carrier (RFC) and biotin, pantothenic acid, and lipoic acid are taken into cells by sodium-dependent multivitamin transporter (SMVT). Drug conjugation to these vitamin moieties can be used to improve intestinal drug absorption and to specifically transport drugs to tissues that have high expression in vitamin transporters.[43]

Another route in regards to transporter-targeted prodrug delivery is through nucleoside transporters usually concentrative (CNTs) and equilibrative (ENTs) nucleoside transporters. These transporters have a general substrate specificity to both purine and pyrimidine nucleosides and, consequently, allow the design of nucleoside-drug conjugates that are likely to have higher levels of cellular uptake. This method is especially applicable to antiviral agents as well as anticancer agents which are acting on intracellular mechanisms.[44]

Capecitabine is a successful non-invasive illustration of bi-step activation of prodrug therapies that uses tumor-selective expression as well as absorption. After the oral administration, the capecitabine is broken down by different enzymes into 5-fluorouracil, and the last stage of activation is done by thymidine phosphorylase that is expressed more in tumor tissue than normal tissue. This design offers improved bioavailability, as well as tumor-selective delivery of drug.[45]

PERMEABILITY-ENHANCED PRODRUG STRATEGIES

1. Lipophilic Prodrugs:

The augmented lipophilicity by means of prodrug development is a traditional yet extremely topical method of raising membrane permeability. Simple fatty acid or alcohol esterification can radically raise lipophilicity to allow passive lipid membrane diffusion. An ideal lipophilicity should be that which takes into consideration, both, the membrane permeation and the solubility in the intestinal fluids.[46]

Figure 4. Prodrug strategies to enhance drug solubility and membrane permeability

An extension of this method is the concept of the use of long-chain fatty acids or phospholipids as promoieties, which results in the concept of lipid-drug conjugates. With these conjugates, they can form micelles, or can react with lipid transporting systems and may gain entry into the body via lymphatic routes bypassing the first-pass metabolism of the liver. Lymphatic absorption can be beneficial especially to drugs that are highly metabolized in the liver or drugs whose action is required in the lymphoid tissue.[47]

Prodrug structure Cyclic prodrug structures would allow them to be more permeable but retain enough aqueous solubility to dissolve. Cyclization causes a decrease in the amount of hydrogen bond acceptors and donors available to interact with aqueous chemistry and leads to higher membrane permeability, and such an effect may be achieved with large lipophilic substituents which are not required. The use of oxazolidinone and lactone prodrug presents this strategy that exhibits better permeability and acceptable solubility properties.[48]

2. Masking Charged Groups:

Ionizable functional groups provide an advantage of solubility in aqueous solution, but can cause extreme depermeabilization of membranes since the cost of transfer of charged species across lipid bilayers is an energetic penalty. Prodrug measures allowing charged groups to be masked in the process of absorption allow the compounds to enter across membranes in neutral form and then restore the active charged species across the cell.[49]

Esters can be used to mask carboxylic groups found in most drugs such as NSAIDs, and antiviral agents. The resulting neutral ester is more permeable and further hydrolysis by intracellular or plasma esterases releases the active form of carboxylates. Tuning of both lipophilicity and activation kinetics can be done by proper choice of the ester substituents.[50]

Amino functional groups in most pharmaceuticals are maskable by means of cognition of carbamate, amide or Mannich base prodrugs. These techniques decrease basicity and hydrogen binding ability and promote membrane permeation. Tertiary amine prodrugs need to strike a balance between increase in permeability and have adequate stability to endure intestinal transit period and the activation kinetics to ensure the drug release occurs at the right time.[51]

Phosphate and sulfate functionalities, even though providing very good water solubility, seriously impair membrane permeation. This is the example of bisphosphonate drugs that are used to treat osteoporosis and exhibit dismal oral bioavailability (usually less than 1 percent). Although classical ester prodrug versions of phosphates are too unstable, other designs phosphoramidate and other novel phosphate prodrug designs have been reported to be useful in selective activation by enzymes.[52]

METABOLIC STABILITY ENHANCEMENT

1. Bypassing First-Pass Metabolism:

Hepatic first-pass metabolism is an important bioavailability limitation to many drugs. This challenge may be countered using prodrug strategies in a number of ways: saturation of metabolic pathways with the prodrug as the parent drug is produced at a regulated rate, prodrug resistance to the metabolic pathway in question and an alternative route to absorption avoiding hepatic metabolism to a degree.[53]

A beta-blocker, propranol, has very high hepatic extraction and hence oral bioavailability is about 30 to 35%. Prodrug methods have pursued several mechanisms of increasing bioavailability such as structures which resist cytochrome P450 hydrolysis and yet will be susceptible to esterase-based activation in systemic circulation upon clearance via the liver.[54]

There is another pathway called lymphatic absorption which can be used to reduce first-pass hepatic metabolism. The lipophilic prodrugs, especially those containing fatty acid compounds, have the ability of being transported in chylomicrons and taken into the systemic circulation via thoracic duct instead of the portal veins. It is especially useful with highly lipophilic (log P > 5) and first pass metabolized drugs.[55]

Figure 5. Prodrug strategies to bypass hepatic first-pass metabolism