INTRODUCTION:

The rectal route though rarely the first choice of drug administration serves as an alternative to oral and invasive administration. Rectal drug delivery is so pivotal when the oral medication is not possible, Intra venous access is not possible and the patients have difficulty in swallowing, nausea and vomiting and for infants or children. Rectal route offers potential advantages for drug delivery. They include: rapid absorption of many low molecular weight drugs, partial avoidance of first pass metabolism, potential for absorption into the lymphatic system, retention of large volumes, possibility of rate controlled drug delivery and absorption enhancement.^1,2 However this route also has some disadvantages: many drugs are poorly or erratically absorbed across the rectal mucosa, a limited absorptin surface area, dissolution problems due to the small fluid content of the rectum, drug metabolism in micro organisms and rectal mucosa. To overcome these, various absorption enhancing adjuvants, surfactants, mixed micelle and cyclodextrins were studied. In this review various absorption enhancers, their effects along with the role of rectal route in delivering various drugs has been discussed. ³

ANATOMY AND PHYSIOLOGY OF RECTUM:

The rectum comprises of the last portion of the large intestine and extends 12 -18cm distally. The rectum has 2 or 3 curves within its lumen created by submucosal folds called the valves of housten.⁴It can be considered as hollow organ with a relatively flat wall surface without vili. The rectal wall is formed by an epithelium which is one cell layer thick and composed of cylindrical cells and goblet cells which secret mucus. The surface area available for drug absorption in the rectum is approximately 200- 400cm².^1,2 Volume of fluid in the rectum is about 1-3ml and is viscous. The rectal mileiu is constant as its p^H is about 7.5-8; the temperature is usually 37°C. Three separate veins comprise the venous drainage of rectum. The superior rectal vein drains the upper rectum and then empties into the rectal vein which flows into the liver. The middle and inferior rectal veins drain the remainder of the rectum and return to the inferior vena cava. Vasculature of the rectum is shown in Figure 1. It is normally empty and filling of rectum provokes a defecation reflex which is under voluntary control. ^{4, 5}

AVOIDANCE OF FIRST PASS ELIMINATION:

Rectal administration of drugs partially avoids hepatic first pass metabolism. The superior rectal vein which peruses the upper part of the rectum drains into the portal vein and subsequently into the liver where as the middle and inferior rectal veins drains the lower part of the rectum and enter into the inferior vena cava and bypass the liver before entering the general circulation. This indicates the drug administered in the lower part of the rectum can bypass the liver resulting in the avoidance of hepatic first pass metabolism and increases systemic circulation. This was investigated and proved with lidocaine, a hepatic high clearance drug. When lidocaine as an osmotically controlled release dosage form, administered low into the rectum resulted in low concentration of first pass metabolite of lidocaine and high concentration of lidocaine. This is an indicative for decreased first pass elimination. The same result was found with proponalol and also increased rectal bioavailability of morphine in man while results with oxeprenolol were disappointing with only half of the bioavailability of the oral preparation. This may be due to the rectal irritation caused by the drug leading to increased peristalsis and defecation. It can be concluded that hepatic first pass elimination can be partially avoided by the drug administration in the lower part of the rectum.⁶

Fig 1: Vasculature of rectum, 1: middle rectal vein; 2: tunica muscularis: stratum longitudinale; 3: m. levator ani; 4: inferior rectal vein; 5: m. sphincter ani externus; 6: superior rectal vein; 7 and 8: plexus venosus rectalis (submucosus); 9: skin; 10: v. marginalis

ABSORPTION OF DRUGS IN RECTUM

Absorption of drugs from rectal epithelium involves two transport routes: the transcellular route and paracellular route. ⁷ An uptake mechanism in transcellular route depends on lipophilicity where paracellular route is drug diffusion through a space between epitelial cells. Rectal absorption of drugs is dependent on several drug characteristics such as partition co efficient and molecular size. Small partition coefficient, large molecular size, charge and high capability of hydrogen bond formation are the typical factors identified for poor absorption of drug. Another significant hindrance is the presence of faeces which also can alter the drug absorption. Solutions, suspensions and suppositories are the common dosage forms for rectal administration of drugs. Rectal contents are generally alkaline and alkaline solutions are quickly absorbed than acid solutions. ⁸ Aqueous and alcoholic solutions are absorbed quickly where as suspensions and suppositories exhibit slow and continuous absorption. Primary methods utilized in improving rectal absorption of the drugs include

· Formulation modification to improve dissolution step of poorly aqueous soluble drugs.

· Modification of the barrier functions of the rectal mucosal membrane.

· Chemical modification of drug to increase partition co efficient. ⁹

ABSORPTION ENHANCERS:

The need to find non invasive routes of protein and peptide administration has focused attention on the other routes of administration. Protein peptide drug delivery by rectal offers the following advantages: low levels of protease activity particularly of pancreatic origin and avoidance of first pas metabolism. However bioavailability of therapeutic proteins and peptides following rectal administration in the absence of absorption enhancers is low. ¹⁰ Various absorption promoting adjuvants with their mechanism of action are discussed here.

Enamine derivatives as absorption enhancers

There have been many investigations of the action of amino acid enamines (phenylalanine and phenylglycine) of beta-diketones (ethylacetoacetate) for enhancing rectal absorption. Because of the rapid absorption of enamine and the chelating capability of ethylacetoacetate and enamine derivatives, research to investigate the feasibility of enamine derivatives of amino acids as absorption-promoting adjuvant has started. Formation of amino acid with ethylacetoacetate in aqueous solution increases with an increase in the pH of the solution.⁹ The enhancing action of enamine derivatives on rat rectal absorption of CMZ (a hydrophilic antibiotic) is dependent on enamine formation capability. The action of the enamines appeared only for short periods due to rapid absorption of the enamines and degradation at the site of action (hydrolysis in aqueous phase). ¹¹

An increase in serum insulin after rectal administration of an insulin suppository containing an enamine occurred rapidly, but the disappearance of serum insulin also occurred rapidly in de-pancreatized dogs. When an enamine suppository without insulin was administered at 20 min after dosing with an insulin suppository containing enamine, an increase in serum insulin concentration was observed after the second administration of the enamine suppository. This result indicates that the time period during which enamines enhance rectal insulin absorption is very short.^12,13 A detailed study of the mechanism for the adjuvant action of enamines has not yet been reported. Because enamines can enhance the rectal absorption of relatively large molecular weight compounds such as insulin and even a stronger action of enamine was observed for relatively small drugs.

A primary concern of enamine derivatives for use in formulation development is the stability of the enamines themselves. In many cases, they undergo rapid hydrolysis at neutral or lower pH values and in suppository bases containing moisture. Acute toxicity data in rats did not show any toxicity up to 1500 mg/kg and furthermore, rapid hydrolysis of enamines may reduce the probability of toxic effects. ^9,11

Salicylates and its derivatives

There are many reports of the utility of salicylate and its derivatives as adjuvants in promoting rectal absorption of hydrophilic antibiotics and polypeptides. The enhancing action of salicylate depends on its concentration at the site of action. It promotes absorption of both low and high molecular weight compounds.¹⁴ Analogues of salicylate such as 5-methoxysalicylate (5MSA) have also been demonstrated as absorption enhancer. Permeability of trypan blue in the form of micro enema has been enhanced in the rectal lumen by salicylate at a concentration of 2%. Reversibility of the effect was clearly shown by exposing the rectal epithelium to salicylate, rinsing out the salicylate after 15 min. and then demonstrating that the epithelium was impermeable to trypan blue. ¹⁵

Rectal bioavailability of antibiotic depends both on the concentration of the adjuvant and on the dosage form. A lipophilic suppository base seems to provide a satisfactory vehicle for the delivery of several antibiotic drugs resulting in good rectal bioavailability. The use of a lipophilic suppository base can easily maintain a high concentration of the adjuvant in the rectal compartment segment in comparison to an aqueous micro enema, because salicylate easily dissolves when dosed as the sodium salt. 5MSA has been reported as stronger absorption enhancer when compared to salicylate. Because melting of a triglyceride base occurred rapidly, the release and dissolution of both the adjuvant and the antibiotics also occurred rapidly in the rectal cavity.^16,17 Both salicylate and 5MSA also increase the rectal absorption of insulin, heparin, gastrin and pentagastrin administered in micro enemas to rats. ^18,19 The action of salicylate in inhibiting cell aggregation may relate to the adjuvant action of salicylate in enhancing the paracellular transport route at high salicylate concentrations. Salicylate may inhibit cell aggregation by two possible mechanisms; i.e. salicylate may modify the cell surface function responsible for binding aresenazo III and/or inactivation of calmodulin by decreasing the Ca²⁺ concentration in the cytosol. ²⁰

Salicylate is transported primarily through epithelial cells at low concentrations, but salicylate is transported via the intercellular route through tight junctions at high concentrations which are reached at therapeutic doses. ^21,22 Salicylate and 5MSA are found to inhibit metabolism from lactate to pyruvate in TCA cycles. Thus, co-administration of salicylate or 5MSA with insulin in a suppository may induce a synergistic therapeutic effect as well as an absorption promoting effect. Salicylate and 5MSA were proved as safe adjuvants by cytological studies. ⁹

Fatty acids

There are many reports about the enhancing action of fatty acids on rectal and small intestinal absorption. Medium chain length fatty acids showed the most effective action as absorption-promoting adjuvants for ampicillin and hydrophilic antibiotics for rectal delivery. It has been reported that the enhancing action of fatty acids is dependent on the partition coefficient. The optimal partition coefficient was calculated at log P = 4.2. This apparent correlation with partition coefficient indicates that the uptake of fatty acids into rectal tissue must be a key factor in their potency as absorption-promoting adjuvants. ²³ With respect to long chain fatty acids such as oleic acid, dosing of oleic acid itself does not cause significant enhancements for the rectal absorption of hydrophilic antibiotics, but when dosed in a mixed micelle formulation prepared with sodium taurocholate, significant absorption promoting action appeared.²⁴ Murakami et al. have reported that the adjuvant action of fatty acids was suppressed by pretreatment with N-ethylmaleimide, a sulfhydryl modifier; i.e. one of the action sites of fatty acids must be the protein fraction of the plasma membrane.²⁵

Strong chelating agents

Strong chelating agents such as EDTA and EGTA have been employed for the study of tight junctions between intestinal epithelial cells. Treatment of the intestinal epithelium by strong chelating agents resulted in loosened tight junctions, which induce an increase in diffusion of solutes through tight junctions. ²⁶ Further, strong chelating agents inhibited the aggregation of isolated intestinal epithelial cells by removing or masking Ca ²⁺ on the surface of the isolated intestinal epithelial cells. EDTA increases the transport of both antipyrine (relatively small molecular weight) and phenol red (relatively larger molecular weight). The transport enhancing action of EDTA was inhibited by ouabain which is a Na⁺, K⁺-ATPase inhibitor.²⁰

Sulf hydryl depleter

Nishihata et al. found that there was a relationship between non-protein thiol content and membrane permeability in isolated rat intestinal cells. When the isolated cells were treated with diethyl maleate (DEM) at high concentrations of 5 mM for a short period of time (before apparent cell death). A significant loss of both non-protein and protein thiols occurred and cell membrane permeability against water soluble antibiotics decreased. Protein thiol loss induced by high concentrations of DEM was inhibited by the co-presence of calmodulin inhibitors in the media of the isolated cells. However, calmodulin inhibitors did not provide for the recovery of the loss of non-protein thiol by DEM. Further, when protein thiol loss by a high concentration of DEM disappeared in the co-presence of a calmodulin inhibitor but non-protein thiol loss continued, the cell membrane permeability against water soluble antibiotics increased again. Thus, non-protein thiol loss induced an increase in cell membrane permeability with respect to water soluble compounds which have relatively small molecular weights.^27,28

Enhancing mechanism of absorption enancers

The mechanisms whereby absorption enhancers improve the absorption of proteins and peptides involve increase in membrane fluidity, expansion of the dimension of the intercellular space, Solubilization of the mucosal membrane, increase in water flux, and reduction of the viscosity of the mucus layer adhering to all mucosal surfaces.²⁹

Efficacy and safety of absorption enhancers

In the study conducted to compare the absorption promoting ability of a poorly absorbed compound (phenol red) and local intestinal damage by different enhancers (sodium glycocholate (Na-GC), sodium taurocholate (Na-TC), sodium deoxycholate (Na-DC), EDTA, sodium salicylate, sodium caprate (Na-CAP), diethyl maleate, n-lauryl-β-D-maltopyranoside (LM), linoleic acid with HCO60 mixed micelle), Na-DC, EDTA and LM proved to be the most effective enhancers. ³⁰

Thus a number of absorption enhancers have been utilized for improving rectal absorption of larger polypeptides and proteins.

Protease inhibitors

Low bioavailability of proteins and peptides by oral route is due to poor membrane permeation characteristics and hydrolysis by digestive enzymes. Therefore, the use of protease inhibitors, which could reduce the degradation of various peptides and proteins due to the inhibition of protease activities at absorption sites, is one of the promising approaches to overcome the delivery problems of these peptides and proteins. Protease inhibitors include aprotinin, trypsin inhibitors, bacitracin. puromycin. bestatin and bile salts such as Na-GC. ³¹ Morishita et al. examined the effect of aprotinin on the hypoglycemic effects of insulin after administration to the duodenum, the jejunum, the ileum and the colon using an in situ loop method and suggested that the effect of aprotinin on intestinal absorption of insulin was site-dependent and the ileum is thought to be suitable for insulin delivery. ³²

From various studies it may be considered that the action of these protease inhibitors to the mucosal membrane was reversible and non-toxic. Consequently, the use of protease inhibitors is one of the most useful approaches to improve the stability and absorption of peptides and proteins, although the mechanisms whereby peptides and proteins were stabilized by these inhibitors were not fully understood. ³¹

Chemical modification of protein and peptide drugs

The absorption enhancers and protease inhibitors can enhance the absorption of normally non-absorbed molecules from the gastrointestinal tract. However, limitations such as local irritation of the mucosa and non-selective absorption of other antigenic compounds are considered drawbacks in the use of absorption enhancers. A potentially useful approach to solve these delivery problems may be chemical modification of peptides and proteins to produce prodrugs and analogues. The intestinal absorption of insulin was known to be very poor due to its extensive degradation by various peptidase and digestive enzymes and poor membrane permeability characteristics. Acyl derivatives of insulin were synthesized to improve the gastrointestinal absorption of insulin. The acyl derivatives of insulin were synthesized by protecting the amino group of glycine-Al, because this group is essential for insulin activity. Chemical modification with fatty acids improved lipophilicity of insulin with increasing carbon numbers of fatty acids attached to the native insulin with increased intestinal absorption and permeation of insulin.

Human calcitonin (hCT) which consists of 32 amino acid residues is one of the calcium regulating hormones. Calcitonin is clinically used for the treatment of increasing bone resorption in metabolic bone disorders such as post-menopausal osteoporosis in older women. Calcitonin was chemically modified with acetic and caproic acids to improve lipophilicity. The lipophilic index value increased with increasing carbon number of fatty acids attached to hCT, indicating that the acylation of hCT enhanced its lipophilicity. The pharmacological activity of hCT was reduced with increasing the carbon numbers of fatty acids introduced to hCT. The half-lives were significantly prolonged by chemical modification with fatty acids in both small and large intestines, indicating that the acyl derivatives were more stable than the native hCT. The extent of the promoting effect was a little higher for a native hCT rather than for acyl-hCT derivatives. These results suggested that it may be possible to achieve the further absorption enhancement of hCT by using a combination of acylation and absorption enhancers.

The acylated peptides with an appropriate lipophilicity may improve stability against various enzymes and may be passively transported by their increased lipophilicity. Furthermore, the acylation also increased the absorption of peptides such as thyrotropin releasing hormone (TRH) and Phe-Gly which are transported by a carrier-mediated process. These findings suggested that lipophilicity may be one of the most important factors for increasing the membrane permeation of drugs. ³¹

Cyclodextrins in rectal drug delivery

Cyclodextrins (CDs), cyclo oligo saccharides consisting of several glucopyranose units are host molecules which form inclusion complex. The three CDs α-CD, β-CD and γ-CD were reported to be useful for rectal drug administration with respect to stabilization, improvement in release and bioavailability and alleviation of local irritation. Parent CDs have been chemically modified to extend physico chemical properties and inclusion capacity of parent CDs. The hydroxyl groups of parent CDs were used as a starting point for chemical modifications of the molecule. Generally, the CD derivatives can be divided into three groups; hydrophilic, hydrophobic and ionizable derivatives. The hydrophilic derivatives included have methylated CDs such as 2,6-dimethyl-β-CD (DM-β-CD) and 2,3,6,-trimethyl-β-CD (TM-β-CD), hydroxyalkylated CDs such as 2-hydroxypropyl-β-CD(HP-β-CD) and branched CDs such as maltosyl-β-CD (G₂-β-CD), which can augment the aqueous solubility and dissolution rate of poorly water-soluble drugs. The hydrophobic CDs include ethylated CDs such as 2,6-diethyl-β-CD (DE-β-CD), which can retard the dissolution rate of water-soluble drugs. In addition, the ionizable CDs include O-carboxymethyl-β-CD (CM-β-CD), O-carboxymethyl-O-ethyl-β-CD (CME-β-CD), β-CD sulfate and sulfobutylether β-CD (SBE-β-CD), which can realize the improvement in inclusion capacity, the modification of dissolution rate and the alleviation of local irritation of drugs, etc, while the hydrophobic CDs may modulate the release of drugs from the vehicles.

Enhanced rectal absorption of lipophilic drugs by CDs is based on the improvement of release from vehicles and the dissolution rates in rectal fluids, Whereas in absorbable drugs such as antibiotics, proteins and peptides is based on the direct action of CDs on the rectal epithelial cells. Many reports have indicated findings that the effects of CDs on the rectal delivery of drugs markedly depends on vehicle type (hydrophilic or oleaginous), physicochemical properties of the complexes and an existence of tertiary excipients such as viscous polymer, etc. prolonged drug release effects of CDs is caused by the sustained release from the vehicles, slower dissolution rates in the rectal fluid or the retardation in the rectal absorption of drugs by an in absorbable complex formation. Various parent CD and derivatives of CDs used in rectal drug delivery are given in Table 1. ³

CDs may improve the chemical stability of drugs in the rectal suppository base. TM-β-CD and DM-β-CD complex with carmofur (a prodrug of 5-flourouracil and apts to hydrolyze to 5-flourouracil) were reported to have greater stability than carmofur in oleagenous suppository bases. The stability of β-CD and DM-β-CD is due to insolubilization of these drugs in the oleagenous suppository base and which may lead to a difficult interaction of drugs with the base. ³³ CDs may inhibit the bioconversion of drugs in the rectum. A combination of α-CD and xanthan gum has inhibited the bioconversion of morphine to glucorides of morphine. The inhibitory effects of α-CD on the glucoronate conjugation of morphine could be ascribed to the inhibition of the upward movement of morphine from areas which are impacted by first pass metabolism. ³⁴

CDs were also reported in enhancing the release of drugs from suppositories. It has been proposed that the enhancing effects of parent CDs on the release of lipophilic drugs from the oleigenous suppository bases could be attributed due to the formation of more hydrophilic complexes of these drugs, because these complexes are of low affinity with the base and rapidly dissolve into the rectal fluids. In comparison with parent CDs, DM-β-CD or HP-β-CD enhance the rectal absorption of lipophilic drugs like EBA to a greater extent. Complexes of β-CD, DM-β-CD, TM-β-CD and ethyl 4-biphenylacetate (EBA) also inhibited the bioconversion of EBA in rat rectal lumen and HP-β-CD has higher potential to improve the rectal absorption of EBA when compared to β-CD and DM-β-CD complexes. ³⁵

There are some reports on the use of CDs as co enhancer. Rectal administration of cefmatazole sodium suppository with inclusion complex of decanoic acid, an absorption enhancer with α-CD complex as an additive increased the plasma concentration significantly than those without additive. ³⁶ However negative effects of CD combination were also reported. The mixture of β-CD and hydroxypropylmethyl cellulose markedly reduced the bioavailability of acetaminophen from both aqueous solution and hydrogels. The lower partition coefficient and the higher hydrophilic property of the β-CD complex and the higher viscosity of HPMC hydrogel matrix might be responsible for the decrease in the rectal absorption. ³⁷ Recently reduction of drug irritancy in rectal delivery by CDs has been reported. HP-β-CD significantly reduced the irritation of the rectal mucosa caused by EBA after the single and multiple administrations of oleaginous suppositories to rats.^35,38

Table 1: The use of cyclodextrins in rectal delivery

CDs	Improvement	Drugs
α-CD	Stability	Morphine hydrochloride
	Release and/or permeation	Cefmetazole Morphine hydrochloride
β-CD	Stability	Carmofur Ethyl 4-biphenylyly acetate
	Release and/or permeation	Naproxen Phenobarbitol Piroxicam
γ-CD	Release and/or permeation	Diazepam Flubiprofen
DM- C	Release and/or permeation	Diazepam Flubiprofen Insulin
	Local irritation	4-biphenylacetic acid Ethyl 4-biphenylyly acetate
TM- β-CD	Release and/or permeation	Diazepam Flubiprofen
HP-β-CD	Release and/or permeation	Diazepam 4-biphenylacetic acid
β-CD polymer	Selective transfer into lymphatics	Carmofur

Rectal administration of the anti epileptic drugs

Therapeutic concentration of the drug must be maintained for optimal seizure control in both acute and chronic treatment of epilepsy. When chronic administration of medication is required but due to lack of oral access and incompatibility of IV formulation of medication alternative routes of drug dosing is required. Importance of rectal route in anti epileptic drug administration is partial avoidance of first pass metabolism. Diazepam by rectal route as solution and gel preparation has been used for several decades for treating repetitive or prolonged seizures in children. Diazepam solution administered rectally result in rapid and complete absorption with peak plasma concentration attained within 5-15 min. the rectal formulation of carbamazepin as a dilute suspension, a viscous gel solution containing equal amount of drug and HPMC and suppositories have been found to be bioequivalent to administration as oral tablets and oral suspension. ^39-43 Evidences show that rectal administration is feasible for lamotrigine, levetiracetam, phenobarbital, topiramate and valproate.⁴⁴

Analgesics by rectal route

Analgesics are probably the most frequently used rectal medications in palliative care. Various opioids have been used rectally to treat cancer pain. Morphine is probably the most useful of these opioids. Morphine when given by oral route undergoes extensive first pass metabolism and has a bioavailability of 30-40%. Administration by the rectal route should theoretically avoid most of the first pass metabolism. In a study conducted by Maloney et al, in terminally ill patients who received various doses of controlled release morphine tablets by rectal route, most of the patients experienced equivalent analgesic action by the rectal route as with oral tablets with no side effects and in few patients the analgesia was maintained without any side effects though the dose of the drug was reduced because of drowsiness.⁴⁵

Babul et al compounded two different preparations of controlled release morphine suppositories a high viscosity and a low viscosity preparation. Both preparations were observed o a greater extent (larger AUC) than oral controlled release tablets but the peak concentrations was slightly lower and the time to peak was significantly prolonged as compared with oral tablets. ⁴⁶ From various studies it was concluded that morphine is reliably absorbed into the systemic circulation when administered rectally to an extent at least that of oral morphine. When oral therapy is not an option, controlled release products are particularly suited for rectal maintenance therapy. Similar observations were found with opioids like oxycodone and methacodoen and also with non steroidal anti inflammatory drugs like raproxem, diclofenac and ibuprofen.⁴⁷

CONCLUSION:

Rectal drug delivery with the advantages of enhancement in drug absorption with enhancers, stabilizing proteins and peptides and partial avoidance of first pass metabolism will undoubtedly be a pioneer in formulation of various challenging compounds. From the safety point of view it will be important to recognize the time window needed for absorption enhancement, problems related to pharmacokinetics and pharmacodynamics of the enhancer and recovery with respect to possible cellular damage.

ABBREVIATIONS:

CMZ: cefmetazole, 5MSA: 5-methoxysalicylate, TCA cycle: tri carboxylic acid cycle, EDTA: ethylene diamine tetra acetic acid, EGTA: ethylene glycol tetra acetic acid, Na⁺, K⁺-ATPase: sodium potassium adenosine triphosphatase, DEM: diethyl maleate, AUC: area under curve.

ACKNOWLEDGEMENTS:

Authors are thankful to Chalapathi Institute of Pharmaceutical Sciences and Ms. K. Lakshmi Prasanna for providing the facilities to bring out this review.

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Received on 01.10.2012 Accepted on 04.11.2012

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