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Combination therapy: A new strategy to manage diabetes and its complications.


Diabetes mellitus (DM) is a metabolic endocrine disorder. It is mainly characterized by hyperglycemia and is associated with the imbalance in carbohydrate, protein and lipid metabolisms (Lin and Sun 2010). It is a chronic disease that arises when the pancreas does not produce enough insulin (insufficient), or when the body cannot effectively use the insulin it produces (Das and Elbein 2006). The International Diabetes Federation estimates that there are over 300 million people around the world with diabetes. This total is expected to reach close to 500 million within 20 years (King et al. 1998; Lin and Sun 2010). Each year another 7 million people develop diabetes. The reasons for this global rise in diabetics are increase in the aging population and increasing trends toward obesity, unhealthy diet and sedentary lifestyle or combination of these (Harris et al. 1998).

There are two main forms of diabetes namely type 1 diabetes (arise due to the diminished production of insulin) and type 2 diabetes (T2DM) (due to impaired response to insulin and [beta]-cell dysfunction) (Kahn 1997). In the case of the former, little or no insulin is being produced. People with T2DM do have sufficient insulin production but the body is not able to use it effectively (Prabhakar and Doble 2008a). Both these types lead to hyperglycemia, excessive urine production, compensatory thirst, increased fluid intake, blurred vision, weight loss, lethargy and changes in energy metabolism. About 20% of the population over the age of 65 have T2DM (Zimmet 2000). In many countries about 5-10% of the total health care budget is used for treating this problem.

Hyperglycemia can cause serious damage to the nerves and blood vessels, the latter leading to macro- and microvascular complications. Three key factors during the onset of hyperglycemia in T2DM are increased hepatic glucose production, diminished insulin secretion, and impaired insulin action (DeFronzo et al. 1992; Lin and Sun 2010; Stumvoll et al. 2005). Insulin resistance refers to suppressed or delayed responses to insulin. It is a problem with the cells that respond to insulin rather than a problem with insulin production itself. Other rare causes of diabetes include pregnancy, due to certain medications, or diseases such as maturity onset diabetes in the young (MODY) (Blanc et al. 2001).

Five classes of oral antidiabetic drugs (OHDs) that are available which work via four different mechanisms are namely those that (i) enhance secretion of insulin in pancreas (sulfonylurea & non-sulfonylurea); (ii) decrease glucose release from the liver (biguanides); (iii) reduce gastrointestinal absorption of carbohydrates ([alpha]-glucosidase inhibitor); and (iv) improve peripheral glucose disposal (biguanides and thiazolidinediones) (Cheng and Fantus 2005). All the drugs are associated with side effects.

Though it is important that glycaemic control should be achieved as rapidly as possible to minimize the impact of glucose toxicity, it is also necessary to provide therapy to control other related risk factors, including oxidative stress, dyslipidaemia, mitochondrial dysfunction, vascular complications, etc. (Duckworth 2001; Jain and Saraf 2010). Hence, a combination therapy becomes necessary to combat the multiple risk factors in diabetics.

World ethnobotanical information reports that about 800 medicinal plants can be used in the control of diabetes mellitus. There are around 450 experimentally proven medicinal plants having antidiabetic properties but complete mechanism of action is available only for about 109 of them (Prabhakar and Doble 2008a,b).

Also, it is equally important to provide sustained glycemic control in the long term to prevent the development of complications (Andersson and Svardsudd 1995). Hence, a judicious selection of suitable agents for combination therapy which can provide most metabolic benefits to the patient with type 2 diabetes should be considered. Thus, a synergistic combination therapy (having more than two pharmacodynamic agents), to treat T2D conditions is necessary. Such a strategy can help to:

(1) Attain rapid and long term glycemic control

(2) Combat other risk factors which arise as a consequence of the T2D

(3) Treat the multiple risk factors that are responsible for the onset and development of T2D

(4) Provide a mild anti-inflammatory effect - since inflammation is both the cause and consequence of T2D condition.

The traditional medicinal plants with various active principles and properties have been used since ancient times by physicians and laymen to treat a variety of human diseases including diabetes, coronary heart disease, and cancer (Alarcon-Aguilara et al. 1998). India has a long history of use of medicinal plants for the management of diabetes. Charaka and Shushruta described in their Charaka samhita and Shushruta samhita the phytophar-macology aspects of diabetes and its complications (Grover and Vats 2001). Medicinal plants have beneficial multiple activities including manipulating the carbohydrate metabolism by various mechanisms, preventing and restoring integrity and functioning of [beta]-cells, controlling insulin release, improving glucose uptake and utilization, and antioxidant properties (Prabhakar and Doble 2008b). Herbal products have been thought to be inherently safe, because of their natural origin and traditional use rather than based on systemic studies (Wood and De Smet 2002). Adverse effects from herbal remedies are reported, but their frequency and severity are unknown. To overcome these drawbacks scientific study of herbal remedies and their potential to cause interactions when used in combination with conventional medicines need to be thoroughly understood and systematically studied (Inamdar et al. 2007).

Nutraceuticals are used as food components (such as vitamins, polyhenols, flavonoids, etc.) and are claimed to have a beneficial effect on health or medical conditions (Kalra 2003). Several of these nutraceuticals have shown to have a beneficial effect on a variety of pathological conditions, including, dyslipidemia, oxidative stress, mitochondrial dysfunction, efc. There is an increasing evidence that in certain pathologic states, the increased production and/or ineffective scavenging of reactive oxygen species (ROS) may play a critical role in many diseases (Robertson 2004). It has been suggested that enhanced production of free radicals and oxidative stress are the main causes for the development of diabetic complications. Nutraceuticals are potent antioxidants and antiinflammatory agents. An overview of the possible targets of the nutraceuticals that could be used in the treatment of T2D and their related complications is given in Table 1.
Table 1 Nutraceuticals in the treatment of T2D (Kelsey et al. 2010;
Mazzio et al. 2011; Omar et al. 2010: Schemmel et al. 2010).

Nutraceuticals with         Target/function   Rationale
 similar activities

[alpha]-Lipoic acid         CNS insulin       Potent inhibitor of
                                              glucose production

Phytosterols, PUFA          CNS leptin        Reduces food intake
                                              Increases gluconeogenesis

Bioactive proteins          CNS GLP1          Reduces hepatic
                                              glucose production

Resveratrol, Quercetin,     AMPK              Reduces hepatic
 chlorogenic acid                             glucose production

Quercetin, Vitamins B6,     Dopamine          Improves insulin
 B12, Folic acid. EPA,                        sensitivity
 [alpha]-lipoic acid,
 L-carnitine, 4-Hydroxy
 isoleucine (Fenugrek)

EGCG, caffeic acid          Serotonin         Improves glucose
                                              tolerance, increases
                                              glucose uptake into cells

EPA, curcumin               Inflammation      Since inflammation and
                                              glucose impairment
                                              are related

Bioflavonoids, Ellagic      Oxidative stress  Since ROS are implicated
 acid, EGCG, Cinnamic acid                    in most of the
 derivatives, Carotenoids,                    metabolic disorders
 [alpha]-lipoic acid,
 vitamin C & E

Garlic constituents,        Blood lipid       Since lipid profile and
 Guggul constituents                          glucose impairment are

Kaempferol, leucocyanidin,  Aldose reductase  Inhibition of polyol
 hesperidin, Apiin                            Pathway

Long term treatment of type 2 diabetes and its complications requires alternation of conventional monotherapy with oral antidiabetic drugs (Jovanovic et al. 2004). Several oral drugs have been studied in combination and have been shown to improve glycemic control when compared to monotherapy. This review discusses the use of drug-drug and drug-phytochemical combinations for the treatment of diabetics. Understanding the effects of one on the other could pave the way for finding effective strategy for treatment of diabetics with less side effects and reduced toxicity.

Combination therapy

Historically, drug combinations have been used for treating diseases and reduce suffering. Attempts have been made to quantitatively measure the dose-effect relationships of each drug alone and in combinations and determine the type of effect of the combination (Bijnsdorp et al. 2011). The choice of the second agent should be based on individual characteristics. Reasonable combinations of drugs include a sulfonylurea plus metformin, a sulfonylurea plus an -glucosidase inhibitor, a sulfonylurea plus a thiazolidinedione, metformin plus repaglinide, biguanide plus [alpha]-glucosidase inhibitor, and metformin plus a thiazolidinedione (Lin and Sun 2010).

Many of the phytomedicines in the drug market are whole extracts of plants because it is known that various components of individual or mixtures of herbs act in synergy and hence are a vital part of their therapeutic efficacy (Lin and Sun 2010). Medicatrix naturae - the power of self-preservation or adjustment has been the motto of traditional medicine and it always prescribes combination therapy (Tiwari and Rao 2002).

Two drugs that produce almost similar effect when used together may sometimes produce enhanced, same or diminished effect. In this interaction there might be three different types of behaviors namely synergy, antagonistic and additive/indifferent. Different types of synergism in combination therapy are possible and they include (Wagner and Ulrich-Merzenich 2009):

(1) Increasing the efficacy of the therapeutic effect,

(2) Decreasing the dosage but increasing or maintaining the same efficacy. Decreased dosage could lead to reduction in toxicity,

(3) Minimizing or slowing down the development of drug resistance,

(4) Providing selective synergism against target (efficacy synergism) versus host (toxicity antagonism).

Enhancing the pharmacokinetic-pharmacodynamic properties

Examples from literature

There are a number of reports which describe the enhancement of the hypoglycemic activity of phytochemicals and synthetic drugs when they are used together in vitro. 8-bromo-cAMP increases the glucose uptake in 3T3-L1 adipocytes when it is used along with prostaglandin F2alpha (PGF2[alpha]) (Chiou and Fong 2005) and endothelin-1 (Fong et al. 2004). This is probably through the enhanced GLUT1 expression via a PKC-dependent mechanism. Endothelin-1 and cAMP, when they are used alone, increase the glucose uptake by 88 & 380%, respectively, but when used in combination they increase the glucose uptake by 1310% with respect to the control (Fong et al. 2004). Similarly PGF2 [alpha] (255% increase) and cAMP (66% increase) when used together increases the glucose uptake by 610% with respect to the control (Chiou and Fong 2005). It is demonstrated that 8-bromo-cAMP enhances the glucose uptake synergistically in adipocytes when combined with aracodonic acid by increasing the synthesis of GLUT1 and acting through PKC dependent pathway (Fong et al. 1999).

Researchers have shown that Phytochemicals when combined together, interact synergistically with each other in in vivo and in vitro conditions (Prabhakar and Doble 2009, 2011a.b,c). Several cinnamic acid derivatives when used in combination with metformin or THZ, increase glucose uptake by L6 myotubes and 3T3-adipocytes. The combinations also increase the expression of genes which are involved in the insulin cascade and decrease the expression of genes involved in the secondary complications such as fatty acid synthase and HMGCoA reducase (Prabhakar and Doble 2011 a,b). Ferulic acid used in combination with these two antidiabetic drugs in the streptozotocin induced diabetic Winstar rats, increases the blood glucose and lipid profile when compared to the diabetic rats. The combination also improves their liver and kidney functions and increases the regeneration of pancreatic [beta]-cells (Prabhakar et al. 2013).

Fibroblast growth factor(FGF)-21, is a novel regulator of insulin-independent glucose transport in 3T3-L1 adipocytes and is reported to have glucose and triglyceride lowering effects in rodent models of diabetes (Moyers et al. 2007). Activation of the PPAR? pathway in 3T3-L1 adipocytes with the corresponding agonist and rosiglitazone (an anti-diabetic drug), enhances the action of FGF-21 to induce tyrosine phosphorylation of FGF receptor-2. Treatment of cells with this combination leads to a pronounced increase in the expression of the GLUT1 glucose transporter and stimulation of glucose transport (Moyers et al. 2007).

GLUT4 is an integral membrane protein expressed only in tissues including fat, skeletal muscles and heart, in which glucose uptake is regulated by insulin. The effect of pioglitazone, ciglitazone and englitazone (members of insulin-sensitizing thiazolidinedione derivatives) on 3T3 cells (Szalkowski et al. 1995); pioglitazone on the expression of glucose transporters GLUT1 and GLUT4 in 3T3-F442A cells (Sandouk et al. 1993a) and; pioglitazone on cellular differentiation and expression of adipose-specific genes namely, adipsin and aP2, in 3T3-F44ZA cells have been reported. The studies indicate that pioglitazone is a potent adipogenic (Sandouk et al. 1993b), and it increases the expression of glucose transporters in 3T3-F442A cells by increasing the stability of the messenger ribonucleic acid transcript (Sandouk et al. 1993b).

Tumor necrosis factor-[alpha] (TNF[alpha]) is a cytokine that stimulates lipolysis, an apparent "dedifferentiation" of adipocytes. It inhibits the transcription of certain adipocyte-specific genes (Weiner et al. 1989). It plays a role in the development of insulin resistance in a variety of catabolic states, including septic shock, acute infection and long-term tumor bearing. Infusion of rats with TNF[alpha] results in marked insulin resistance. Thiazolidinediones have been used to check the inhibitory effect of TNF-[alpha] on the differentiation of 3T3-Ll adipocytes cells. Long term incubation of these adipocytes with TNF-[alpha] shows a reduction in glucose uptake in response to insulin. Whereas, GLUT4 and other specific adipocyte genes are ameliorated by thiazolidinedione treatment (Diaz-Delfin et al. 2007).

A clinical trial with 233 T2DM patients has been reported to access the effects of combination therapy with TZD and exenatide. Larger reductions in FPG or PPG were observed in the TZD plus exenatide group when compared to TZD or placebo alone. A significant reduction in body weight was also observed in this group. The function of [beta]-cell increased by 19% for those who received the combination and it decreased by 6% for those in the placebo group (Schwartz 2008). Several pharmaceutical companies are marketing a combination of two or three different types of OHDs (Table 2) to get a significant improvement in the management of blood glucose and secondary complications.
Table 2 Examples of marketed drugs prescribed in combination for the
treatment of diabetes.

Name          Drug combinations             Manufacturer
Clucovance    Glyburide (sulfonylurea)+     Merck Serono

Metaglip      glipizide (sulfonylurea) +    Bristol-Myers Squibb

Benformin     Glibenclamide + Metformin     Orchid pharma

Claformin     Gliclazide + Metformin        Orchid pharma

Glimitide     Glimepiride+ Metformin        Orchid pharma

Piocon Forte  Pioglitazone + Metformin      Orchid pharma

MetbeticG     Gliclazide + Metformin        Cadilla pharmaceuticals

Glyloc M      Gliclazide + Metformin        Cadilla pharmaceuticals

Glista Ml     Glimepiride + Metformin       Cadilla pharmaceuticals

Piozulin      Pioglitazone + Metformin      Cadilla pharmaceuticals

Metact        Pioglitazone + Metformin      Takeda pharmaceuticals

Glista PM2    Glimepiride + Pioglitazone +  Cadilla pharmaceuticals

PioconGM 1    Glimepiride + Pioglitazone +  Orchid pharma

PioconGM 2    Glimepiride + Pioglitazone +  Orchid pharma

Persons with diabetes often have a number of coexisting health problems. So in addition to diabetic medications pills, other drugs are often needed to control these problems. These drugs include statins to control high cholesterol, diuretics or beta-blockers to control high blood pressure, antidepressants to manage depression or neuropathy pain and aspirin to prevent a heart attack. Medications taken simultaneously could lead to drug-drug interactions. This interaction refers to the modification in the effect of a drug when administered with another drug, which may increase or decrease the action of either substance. The interaction may lead to change in the quantity of either of the drugs absorbed by the body or an alteration in the ability of either of the drug to bind to the receptor sites (De Sousa et al. 2004).

There are some drugs which have the potential to raise blood glucose and lead to inefficacy of an oral hypoglycaemic drug. Patients who are administered high dose of corticosteroids may need also insulin to control their blood glucose. Stopping a drug which causes hyperglycaemia may require a parallel reduction in the dose of a hypoglycaemic drug.

Some drugs chemicals or phytochemicals can enhance the blood glucose level whereas some can lower blood glucose but their mechanism of action is not well understood (Tables 3 and 4). These compounds when used along with other antidiabetic drugs might cause hyperglycemia or hypoglycaemia depending upon their effect on the blood glucose level. So the patient may need to modify the drug dose according to the situation in consultation with his physician. Conversely stopping a drug with the potential to lower blood glucose might produce relative inefficacy of a hypoglycaemic drug and may create a need for its increased dose (Shenfield 2001). [beta]-blockers can mask the warning signs of hypoglycaemia, and the use of non-selective [beta]-blockers may impair the normal recovery reaction to hypoglycaemia.
Table 3 Medications that can raise blood glucose.

Drug                               Mechanism of action

Clonidine                          Adrenergic action

Clozapine                          Impairs insulin secretion

Corticosteroids                    Oppose insulin action

Diuretics (especially thiazides)   Oppose insulin action

Nicotinic acid                     Opposes insulin action

Nifedipine (but not other calcium  Delays insulin action

Oral contraceptive hormones        Oppose insulin action

Phenytoin                          Blocks insulin secretion

Phenothiazines                     Not known

Sugar-containing syrups (e.g.      Increased glucose intake
 antibiotics/cough mixtures)
Table 4 Medications that may lower blood glucose.

Drug                          Mechanism of action

ACE inhibitors                Increase insulin action

Alcohol                       Inhibits hepatic glucose
                              production and release

Fibrates                      Not known

Monoamine oxidase inhibitors  Not known

Quinine (quinidine)           Increases insulin secretion

Salicylates (large dose)      Not known

Pharmacokinetic interactions

The interaction of various classes of drugs leading to change in their pharmacokinetic behavior is discussed below.

Sulfonylureas: They are used primarily for the management of diabetes mellitus and they are ineffective in the case of type 1 diabetes or pancreatectomy. Some sulfonylureas are metabolized by liver metabolic enzymes i.e., cytochrome P450 (De Sousa et al. 2004). Person with impaired renal function can accumulate this drug leading to hypoglycemia. Gluburide has short half-life but prolonged biological effect due to the formation of active metabolites which are also metabolized in liver similar to chlorpropamide. Gly-buride binds strongly to plasma protein and is recycled hepatically.

Antacids increase the absorption of sulfonylurea drugs and hence may produce higher peak concentrations leading to temporary hypoglycaemia (Lin and Sun 2010). Sulfonylureas are generally protein bound drugs and may get displaced from blood proteins by drugs such as sulfonamides, coumarines, probenecids, nonsteroidal anti-inflammatory drugs. This can cause a short-term increase in unbound free sulfonylurea and hence cause temporary hypoglycaemia (Diaz-Delfin et al. 2007).

Repaglinide: These belong to the class of non-sulfonylurea insulin secretogogues. They have several desirable properties including a rapid onset and short duration of action, and metabolism and excretion by non-renal routes. They can work synergistically with other antidiabetic drugs such as metformin in patients whose hyperglycemia is not controlled by monotherapy. Repaglinide metabolism may be decreased by inhibitors of CYP 3A4 such as the azole antifungal (ketoconazole, miconazole, etc.) and some antibiotics including erythromycin, resulting in increased serum concentrations. Drugs that induce the CYP 3A4(i.e. troglitazone, rifampin, barbiturates, carbamazepine) may increase repaglinide metabolism and thereby decrease its antidiabetic effect when used concurrently (Table 5).
Table 5 Potential interactions between sulfonylureas or repaglinide
and drugs which alter hepatic enzymes.

Inducers of metabolism (reduce  Inhibitors of metabolism (increase

concentration of hypoglycaemic  concentration of hypoglycaemic
drug)                           drug)

Phenytoin                       Allopurinol(a)

Phenobarbitone                  Chloramphenicol

Rifabutin                       Cimetidine(a)

Rifampicin                      Erythromycin, 'Azole' antifungals
(a) Repaglinide concentrations not increased.

Metformin: Metformin is not metabolized at all but is completely excreted in urine. So it may accumulate and cause lactic acidosis if other medications are used together which induce renal failure (Grover and Vats 2002). In noninsulin-dependent diabetes mellitus patients whose hyperglycemia is not adequately managed with a sulfonylurea (glyburide) and diet, addition of metformin improves glycemic control, reducing fasting plasma glucose levels by 20%. The synergistic effect is due to the fact that they have different mechanism of action. Biguanides may increase the risk of lactic acidosis in patients with a history of alcoholism, liver disease, renal disease, pregnancy and lactation and CHF or patients with CV disease and chronic cardiopulmonary disease. Metformin should be stopped for at least two days prior to use of radiographic dyes since the latter may be hyperosmolal and induce dehydration, thereby increasing the incidence of acidosis.

Acarbose: It is a [alpha]-glucosidase inhibitor type of drug which inhibits [alpha]-glucosidase in the gut wall and so the conversion of polysachharide and oligosachharide to monosachharide and as a result the absorption (Shenfield 2001). Acarbose does not exert any direct effect on insulin resistance in humans. Because of its different mechanism of action, it enhances glycemic control when it is used in combination with sulfonylureas. It decreases the insulinotropic and weight increasing effects of the sulfonylureas. Acarbose interferes with metformin absorption so generally these two should not be used in combination.

Thiazolidinediones (TZD): Troglitazone, one out of the three TZD, has been withdrawn worldwide because of its high hepato-toxicity (Shenfield 2001). They work as Peroxisome Proliferator activated receptor (PPAR)[gamma] agonists. The PPARs have been identified as key regulators of carbohydrate and lipid metabolism, because they stimulate protein synthesis in a wide variety of processes including energetic metabolism, proliferation and cellular differentiation. The TZD are compounds that increase the tissues sensibility (muscle, adiposity tissue, and liver) to the insulin action, so they are used in the treatment of T2DM. These drugs produce several adverse effects, such as weight increase, edema, anemia, pulmonary edema, and congestive cardiac failure (Bermudez et al. 2010). It also induces cytochrome P450 3A4 and interacts with a number of drugs including cyclosporin and oral contraceptives. Pioglitazone is also known to induce the same enzyme which may lead to similar problems.

Drug-phytochemical interaction

Plant extracts and their ingredients could be a more effective strategy for the management of diabetes mellitus because of the likelihood of high compliance. They are largely free from side effects, have better effectiveness, act on multiple target sites and are of relatively low cost (Alarcon-Aguilara et al. 1998). When using a combination therapy, the dosing of both the compounds is very important. These combinations can show one type of behavior at one concentration mixture and other type at another concentration. So in such studies a perfect combination of phytochemicals and commercial oral antidiabetic drugs should be used (Prabhakar and Doble 2009, 2011a,b,c; Prabhakar et al. 2013). Experiments comparing single isolated compound with original extract have confirmed that many plant constituents, primarily phenolics and terpenoids, exert polyvalent pharmacological effects (Wagner and Ulrich-Merzenich 2009). Herbs, vitamins, and other dietary supplements may modify, magnify or oppose, the action of drugs when taken together. As with many drug-phytochemical interactions, the information for many dietary supplements is scarce (Xu et al. 2008). Deleterious effects are most pronounced when anticoagulants, cardiovascular medications, oral hypoglycemics, and antiretroviral drugs are used in combination with dietary supplements. Herbal medicines are ubiquitous but data on herb-drug interactions are very limited (Table 6). It is generally believed that the use of herbs with medicine enhances the effect of the latter and reduces its adverse effects (Lin and Sun 2010). Ginseng may be used to treat diabetes, but may interfere with drugs including warfarin and prevent blood clotting.
Table 6 Examples of drug-phytochemical combination which affect
glucose uptake.

Drug               Phytochemical         Effects

Rosiglitazone      Momordicha charantia  Decrease blood glucose,
                                         increase the volume of islet
                                         [beta]-cells and prevent
                                         hepatic damage

Metformin          Arecoline             Increase glucose uptake
                                         in vitro by cells

Metformin          Caffeic acid          Increase glucose uptake
                                         in vitro by cells

Metformin          Chlorogenic acid      Increase glucose uptake
                                         in vitro by cells

Metformin          Coumaric acid         Increase glucose uptake
                                         in vitro by cells

Metformin          Eugenol               Increase glucose uptake
                                         in vitro by cells

Metformin          Ferulic acid          Increase glucose uptake
                                         in vitro by cells, decrease
                                         blood glucose, improve lipid
                                         profile and increase [beta]-
                                         cell regeneration and its mass
                                         in [beta]-cells damaged rats

Thiazolidinedione  Arecoline             Increase glucose uptake
                                         in vitro by cells

Thiazolidinedione  Caffeic acid          Increase glucose uptake
                                         in vitro by cells

Thiazolidinedione  Chlorogenic acid      Increase glucose uptake
                                         in vitro by cells

Thiazolidinedione  Coumaric acid         Increase glucose uptake
                                         in vitro by cells

Thiazolidinedione  Eugenol               Increase glucose uptake
                                         in vitro by cells

Thiazolidinedione  Ferulic acid          Increase glucose uptake
                                         in vitro by cells, decrease
                                         blood glucose, improve lipid
                                         profile and increase [beta]-
                                         cell regeneration and its mass
                                         in [beta]-cells damaged rats

Combination of rosiglitazone with Momordica charantia enhanced the hypoglycemic in streptozotosin induced adult and neonatal diabetic rats (Nivitabishekam et al. 2009). Histopatho-logical studies of these rats revealed that administration of both the compounds together increased the volume of the islet cells in pancreas and prevented the hepatic damage when compared to the control. Pretreatment of L6 myotubes with subtherapeutic dose of sodium orthovanadate or vanadyl sulfate and mTOR induced rapamycin lead to 5- and 3-fold increase in insulin induced glucose uptake respectively (O'Connor and Freund 2003). Vanadium and rapamycin synergise to enhance the glucose uptake by preventing the loss of IRS-1 and the decay of IRS-l/PI3Kinase complex (O'Connor and Freund 2003).

St. John's wort (Devil's claw; Harpagophytum procumbens)

The behavior of St. John's wort in combination with drugs has been well documented. Its extract reduces blood glucose in fasted normal and diabetic animals (Lin and Sun 2010). Metformin (Glu-cophage) which is used as a treatment for hyperglycemia in people with T2DM fail to perform effectively if it is combined with this natural product. Sulfonylureas such as Glyburide (Diabeta, Gly-nase, Micronase), Glipizide (Glucotrol), Glimepirid (Amaryl) help to treat T2DM by increasing the insulin release from the insulin storage vescicles. These drugs also may not be effective when used in combination with this extract.

Garlic (Allium sativum). Bitter melon (Momordica charantia) and Asian ginseng (Panax ginseng, P. quinquefolium) when combined with hypoglycemic drugs appears to potentiate the action of the latter (Dey et al. 2002). Many reports of this kind lack systematic data on concomitant drug use. Also no data is available on the composition of the herbs used, and identify the effect of contaminants present in them.

Phytochemicals such as ferulic acid, chlorogenic acid, caffeic acid, coumaric acid, cinnamic acid, eugenol, berberine have shown synergistic interaction with commercial oral antidiabetic drugs when they are tested in two different cell lines at basal level (Prabhakar and Doble 2009, 2011 a,b,c). Most of these phytochemicals act at various locations in the insulin signaling cascade and improves their effectivity. They also shown to reduce the lipid content through regulation of their synthetic pathways.

A recent study shows that ferulic acid, a phytochemical present mostly in cell wall of a number of plants, when used in combination with commercial oral antidiabetic drugs like metformin and thiazolidinedione have an synergistic activity. These combinations have better hypoglycemic and hypolipidemic compared to when they have used alone in diabetic rats (Prabhakar et al. 2013). These combinations also show an increase in the number of pancreatic beta cells in streptozotocin induced diabetic rats.

Cocrystal of metformin with a nutraceutical [WO/2010/134085] showed enhanced bioavailability of the former with an additional advantages of a having a mild anti-hypertensive action due to the presence of the latter (Reddy et al. 2011). This new Composition of Matter (COM) is being positioned for the T2D patients who are developing the co-morbidity of hypertension. The nutraceutical is less toxic. Additionally, this cocrystal shows significant insulin sensitization action and marked enhancement in the residence time of metformin in Wistar rats (Fig. 1).


This study indicates that synergistic combination of APIs with nutraceuticals, is definitely the way forward for an effective and efficient therapy for T2D. In addition, various formulation methods (microencapsulation, cocrystallization, nanocrystallization, micronization, etc.) would further enhance the bioavailability and thus result in an ideal medication (Junghanns and Muller 2008).

Evaluation of interaction in diabetes

Evaluation of the interaction between commercial drugs or between drug and natural product is important while practicing combination therapy. Several methods are reported to detect synergy between antibacterial or antifungal agents. These includes FIC index. Time kill curve, checkerboard and E test (Hemaiswarya and Doble 2009, 2010; Hemaiswarya et al. 2008). Two methods namely, isobologram (Tallarida 2001) and combination index (Zhao et al. 2004) have been mentioned in the literature to determine synergy while using combination therapy in treating diabetics.


The first method uses an isobologram, which is a graphical representation of the interaction of two chemicals (drugs, phytochemicals or combination of both). The isobologram was introduced earlier (Loewe 1953, 1957) to study the combination of ethyl alcohol and chloral hydrate (Gessner and Cabana 1970) and was suitably adapted in this reference to determine the uptake of 2-deoxyglucose in vitro by L6 myotubes (Prabhakar and Doble 2009, 2011c) or 3T3 adipocytes (Prabhakar and Doble 2011 a,b). The X-and Y-axes of the isobolograms (Fig. 2) are the concentrations of the two compounds A and B required individually, and in combination, to bring out the same level of 2DG uptake by the cells.


The straight line connecting the individual concentration (Fig. 2A and B) required to achieve the same level of 2DG uptake is termed as the "line of additive". If the combination data is below this line then it is termed as "synergistic" and on the other hand if it is above this straight line then it is termed as "antagonistic" (Tallarida 2001).

Combination index

Another method to explain the interaction of two compounds is based on a calculated constant known as combination index (CI). It is estimated as

Combination index(Cl) = [C[.sup.a]/IC[.sup.a]]+[C[.sup.b]/IC[.sup.b]]

where C[.sub.a] and C[.sub.b] are the concentrations of compounds A and B used together to achieve a fixed effect (in this case certain amount of glucose uptake by the cells). IC[.sub.a] and IC[.sub.b] are the concentrations of A and B, respectively required individually to achieve the same effect. A CI value of less than, equal to, and more than one, indicates synergy, additivity, and antagonism respectively between these two compounds (Zhao et al. 2004; Ting-Chao 2010).


The market for herbal medicines is booming and the evidence for their effectiveness is growing, but inadequate regulations and absence of proper standards have hampered their use. Therefore product standardization and understanding the efficacy, safety and therapeutic risk/benefits associated with the use of herbals need to be properly evaluated. The mechanism of action of these are not known (Tiwari and Rao 2002).

Many of the effective phytomedicines available in the market is whole extracts of plants and it is believed that synergistic interaction between the various constituents in the mixture is vital for their therapeutic efficacy (Williamson 2001). Combination therapy can be extended in the treatment of several disorders including inflammatory, stress-induced insomnia and osteoarthritis. Today illnesses such as cancer, AIDS, hypertension, tuberculosis etc., are successfully treated with combination of 3-5 synthetic drugs. Multitargeted therapy approach involving the application of phytochemicals or plant extracts and synthetic drugs as anticancer agents has been well documented (Hemaiswarya et al. 2008). There are several examples of herb extract showing a better effect than an equivalent dose of an isolated compound. The reason for this synergy could be enhanced bioavailability, cumulative effects or additive properties of the constituents (Manek et al. 2011).

There are a number of reports available on the interaction of hypoglycemic drugs in vitro and in clinical trials. But there are no reports on the use of natural products (phytochemicals or neu-traceuticals) and synthetic drug combinations in clinical settings. As discussed in the review, there are possibilities of using natural products in combination with antidiabetic drugs as hypoglycemic agents. The undesirable side effects of the latter could be reduced by decreasing the dose of the synthetic drugs (Lin and Sun 2010). In order to select a phytochemical that could act in synergy with a drug it is necessary to understand the complete molecular mechanism of the drug action in the presence and absence of the former. The problems that still need to be addressed are stability, selectivity and bioavailability of these natural products, closing strategy and any adverse herb-drug interaction. The maximum benefit from these combinations can be achieved when the pharmacokinetics and pharmacodynamics of these two compounds match. The optimal dosing concentration for the combination should be properly determined and verified with the use of animal models. Proper precaution and care should be taken to avoid the severe hypoglycemia that may occur due to use of combination. Also, preclinical studies should be performed with pure phytochemicals.

[c] 2013 Elsevier GmbH. All rights reserved.


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P.K. Prabhakar (a), Anil Kumar (b), Mukesh Doble (c), (*)

(a) Lovely Faculty of Applied Medical Sciences, LPU, Phagwara, Punjab, India

(b) Tata Chemicals Ltd., Innovation Centre, Pirangut, Pune 412108, India

(c) Department of Biotechnology, IIT Madras, Chennai, Tamilnadu, India


Article history:

Received 5 April 2013

Received in revised form 18 July 2013

Accepted 20 August 2013

* Corresponding author. Tel.: +91 4422574107; fax: +91 4422574102. E-mail addresses:, (M. Doble).
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Author:Prabhakar, P.K.; Kumar, Anil; Doble, Mukesh
Publication:Phytomedicine: International Journal of Phytotherapy & Phytopharmacology
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Date:Jan 15, 2014
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