FormalPara Key Summary Points

Self-microemulsifying drug delivery systems (SMEDDS) allow for new formulations of lipophilic drugs with improved bioavailability and pharmacokinetic profiles without compromising efficacy.

SMEDDS technology differs from other emulsions mainly in thermodynamic stability, size, and surfactant/lipid composition.

SMEDDS drug formulations are increasingly used in the clinic, including the recent Food and Drug Administration (FDA)–approved celecoxib oral solution for the acute treatment of migraine.

Celecoxib oral solution is a selective cyclo-oxygenase-2 inhibitor designed as an acute, low-dose, oral-solution SMEDDS formulation of celecoxib indicated for and recently recommended by the American Headache Society to treat migraine attacks acutely.

Celecoxib oral solution has an advantageous pharmacokinetic profile, allowing for a lower cumulative dose and a potentially lower risk of adverse events than with non-SMEDDS celecoxib formulations, thereby demonstrating the clinical utility of a SMEDDS formulation in the acute treatment of migraine.

Background

Drugs are most often administered orally due to ease of use and widespread patient acceptance [1]. However, orally administered drugs may display variable pharmacokinetics (i.e., absorption) [1,2,3]. The gastric lumen is highly acidic and filled with digestive enzymes that can degrade drugs before they reach their site of absorption in the small intestine [1]. The low gastric pH can reduce drug solubility and drug dissolution [1,2,3], and some orally administered drugs irritate the gastrointestinal (GI) tract [1, 4]. Further, approximately 40% of polar hydrophobic compounds and other macromolecules with low aqueous solubility are not effectively absorbed through intestinal epithelial cells [1,2,3]. Often, we administer these drugs at higher doses to overcome inefficient absorption or first-pass hepatic metabolism. Despite these challenges, oral drug delivery remains the preferred route of administration.

To surmount the above oral drug delivery obstacles, several improvements and lipid-based formulations have been introduced, such as emulsions, microemulsions, solid-lipid nanoparticles, and lipoplexes, that can optimize solubilization and transport of hydrophobic compounds [3].

Hoar and Schulman first described microemulsions in 1943; they have been used in many applications since, including cosmetics, beverages, and solubilized drugs [5]. Unlike emulsions, which will eventually undergo phase separation due to their instability, microemulsions are a thermodynamically stable liquid–solution system of water, lipids, and amphiphiles (e.g., surfactant and co-surfactant) [3, 6, 7].

Here, we will focus on the self-microemulsifying drug delivery system (SMEDDS)—and what differentiates it from the analogous self-emulsifying drug delivery system (SEDDS) and the self-nanoemulsifying drug delivery system (SNEDDS)—as a vehicle for poorly soluble drugs. This manuscript is based on previously performed and published studies and does not contain any new human participant or animal studies performed by any of the authors.

Components of SMEDDS

A SMEDDS microemulsion formulation is composed of lipid, drug, surfactant, co-surfactant, and solubilized drug in water [2, 3, 8]. The property of self-emulsification depends upon the specific oil and surfactant pair, the surfactant concentration, and the oil-to-surfactant ratio [2, 3, 8, 9].

The type of lipid used in SMEDDS dictates the amount of drug that can solubilize, and it provides access to lymphatic absorption [2, 3, 8]. Hydrophobic drugs solubilize more effectively in synthetic oils (e.g., hydrolyzed vegetable oils) or digestive lipids (e.g., diglycerides and fatty acids) [2, 9,10,11]. Medium-chain triglycerides (MCTs; with 6–12 carbons) and long-chain triglycerides (LCTs; with > 12 carbons) with different degrees of saturation are commonly used [2]. Compared with LCTs, MCTs self-emulsify well, oxidate less, and are digested more efficiently with better fluidity and solubility [2, 3, 12]. However, MCTs are absorbed primarily into portal blood and systemic circulation, whereas LCTs are incorporated primarily into chylomicrons and then freely transported via intestinal lymphatics [3, 13]. Lipid concentration may also affect drug absorption through the lymphatic system. A microemulsion of sirolimus containing > 25% MCT oil content showed enhanced lymphatic drug absorption, with 55–90% of the drug absorbed lymphatically [2, 14].

Ideal drug candidates for incorporation into a SMEDDS formulation should be sufficiently hydrophobic with a high octanol–water partition coefficient (log P > 5) [2, 3]. This means that the drug should be more than five times more likely to dissolve in the hydrophobic octanol than in water. The more hydrophobic a drug is, the more effectively it emulsifies in SMEDDS [2, 15]. Liquid droplets form due to the surface tension of liquids, which is related to molecular attractive forces between the molecules, including hydrogen bonding [16]. In a bulk liquid, the same attractive forces affect a molecule from all sides, whereas at the surface this molecular attraction is lacking from one direction. Surface energy and surface tension are caused by these asymmetrical forces [17]. Thus, surface tension is a manifestation of the sum of cohesive forces affecting a liquid. Similarly, the surface free energy at the interface between two poorly miscible or immiscible liquids is the source of interfacial tension. Two liquids are immiscible if there are substantive differences in cohesion forces between them [17].

The primary function of a surfactant in a SMEDDS formulation is to lower the interfacial surface tension to a negative value, which is necessary for spontaneous self-emulsification [2, 3, 9]. Effective surfactants have higher hydrophobic–lipophilic balance values (> 12) in the range of solubilizers and detergents [2, 3, 18]. Surfactants also help solubilize the drug in a SMEDDS formulation and skip the dissolution phase associated with other drug formulations [19]. High concentrations of certain surfactants may lead to GI irritation; thus, optimization of excipients is important in minimizing toxicity [10, 20]. We more commonly use non-ionic surfactants than ionic surfactants in SMEDDS formulations due to their decreased GI toxicity [21].

In a SMEDDS formulation, the co-surfactant facilitates dissolution of both the drug and the surfactant in the lipid phase by allowing more water into the microemulsion [2, 3]. A co-surfactant imparts flexibility and further reduces interfacial tension, which contributes to self-emulsification [2, 3, 21]. We often successfully employ short- and medium-chain alcohols, such as polyethylene glycol and propylene glycol, in SMEDDS formulations as co-surfactants [21, 22].

Surfactants and co-surfactants represent the largest proportion of a SMEDDS formulation. A stable SEDDS is formed with approximately 30–60% surfactant composition [9], whereas SMEDDS and SNEDDS are more likely to form with higher surfactant-/co-surfactant-to-oil ratios (Table 1) [9, 23]. For example, the SMEDDS formulations of celecoxib oral solution (ELYXYB™) and ticagrelor (Brilinta®) comprise approximately 50% surfactant and 35–40% co-surfactant [23, 24].

Table 1 Characteristics differentiating SEDDS, SNEDDS, and SMEDDS
Full size table

Differentiating SEDDS, SNEDDS, and SMEDDS

SEDDS, SNEDDS, and SMEDDS form oil-in-water emulsions of decreasing droplet sizes when introduced into aqueous phases with gentle agitation, for example, in the GI tract [25, 26]. The three delivery systems utilize similar technology, differentiated, however, by composition, size of the drug-delivery vehicle droplets, and thermodynamic stability (Table 1).

Unless otherwise specified, SEDDS refers to any self-emulsifying system, including those with larger droplets (> 400 nm) [9, 25, 27], which tend to be opaque and incorporate less surfactant and more lipid than SMEDDS or SNEDDS [3, 28]. Due to the larger size of the droplets, SEDDS depends more on bile salts than SMEDDS for droplet formation and to facilitate drug absorption [2].

SMEDDS and SNEDDS share several characteristics compared with SEDDS, including optical transparency, increased absorption, and less dependence on bile salts to emulsify droplets [2, 28, 29]. Although microemulsions (< 100 nm) tend to have smaller diameters than nano-emulsions (≈ 200 nm) [21, 30, 31], the main differentiating characteristic is that SMEDDS is thermodynamically stable and SNEDDS is not [2, 3, 30]. A SMEDDS formulation may remain kinetically stable indefinitely, provided it is not diluted or its temperature is not significantly changed [30, 32]. Upon selection of appropriate excipients, SNEDDS formulations can be relatively kinetically stable for periods of months [29, 30, 33, 34]. Further, SMEDDS create microemulsions regardless of the order of preparation by utilizing intrinsic chemical properties of the formulation, rather than the special mechanical mixing and handling required to form SNEDDS or SEDDS [3, 30, 31].

Benefits of SMEDDS

Several advantageous characteristics of SMEDDS formulations help facilitate absorption and bioavailability of hydrophobic drugs [2, 35]. In the GI tract, a drug in a SMEDDS formulation easily separates from the vehicle because it remains in a solubilized form [2, 21]. This circumvents the variable drug dissolution process in the GI tract [2, 25]. Drug dissolution rates increase and remain unphased by varying GI pH [36,37,38]. The small droplet sizes of a SMEDDS formulation provide ample surface area for efficient absorption, contributing to more consistent serum drug levels [20, 21, 25]. SMEDDS droplets allow for structural changes in the GI tract that permit their absorption as microemulsions, in micelles via endocytosis, or via diffusion through the loosening and opening of epithelial tight junctions [10, 19, 35, 39]. The microemulsion oil droplets also protect the drug from chemical or enzymatic catabolism in the GI tract as the drug is being presented to the body in oil droplets [2, 9]. Drugs delivered in SMEDDS formulations can circumvent first-pass hepatic metabolism by promoting lymphatic uptake; the drug is absorbed into a chylomicron, directly increasing uptake into the lymphatic system [3, 11, 25, 40]. Co-administration of a SMEDDS formulation with food does not affect drug absorption, unlike with other non–lipid-based formulations, but rather alleviates the food effect on absorption [3, 41]. Due to its thermodynamic stability, a SMEDDS formulation also offers an indefinitely long shelf life. Further, the relatively simple manufacturing process involved in SMEDDS formulations requires less energy than other emulsions, due to their self-emulsifying properties [21].

SMEDDS Improves Pharmacokinetics in Drug Reformulations

We have effectively employed SMEDDS technology to enhance pharmacokinetic (PK) properties of drugs for decades. In 1996, cyclosporin A was formulated as a dissolvable SMEDDS tablet (Sandimmune Neoral®), which showed approximately 200% increased bioavailability over a previous oral formulation [19, 42,43,44,45]. A SMEDDS formulation of ritonavir (Norvir®) showed improvements in the plasma profile in terms of maximum plasma concentration (Cmax) and area under the concentration–time curve (AUC0–24 h), which were nearly twofold higher than a comparable aqueous suspension of ritonavir [46]. With some drugs, SMEDDS technology increases PK performance by orders of magnitude, as evidenced by a paclitaxel (Taxol®) SMEDDS formulation, which increased bioavailability by fivefold and Cmax by nearly tenfold, compared with a non-SMEDDS formulation [47]. A SMEDDS formulation of saquinavir (Invirase®) showed similar dramatic bioavailability improvements over a non-SMEDDS formulation, with a fivefold increase in AUC0-∞ and a 13-fold increase in Cmax [48]. Recent clinical applications of SMEDDS technology include reformulations of antihistamines and nonsteroidal anti-inflammatory drugs (NSAIDs). A SMEDDS formulation of loratadine increased Cmax by fourfold and oral bioavailability in terms of AUC0–∞ by ninefold, compared with the non-SMEDDS formulation [49]. Ibuprofen in a SMEDDS formulation showed similar increases in bioavailability (3.4-fold increase) and Cmax (3.8-fold increase), compared with ibuprofen in suspension [50]. Together, these data support the advantageous PK profile and increased bioavailability of drugs solubilized in SMEDDS.

Some routes of drug administration may be uncomfortable or carry associated stigma, such as injected or intranasal treatments. An added benefit of SMEDDS formulations is that they are available in patient-friendly gelatinized capsules, tablets, or oral solutions [3, 21]. Although SMEDDS tablets often dissolve upon ingestion, some capsule formulations may be more cumbersome and difficult to swallow. Liquid oral solutions are easier to administer without water or food.

SMEDDS in the Treatment of Migraine: Celecoxib Oral Solution (Elyxyb™)

The American Headache Society (AHS) published their consensus statement for migraine in 2021 [51]. The consensus statement included no major changes but did incorporate new acute migraine therapies, including oral tablets, capsules, and sachets of NSAIDs, triptans (serotonin [5-HT] receptor agonists), a ditan (lasmiditan), and gepants (small-molecule calcitonin gene-related peptide [CGRP] antagonists) [51]. The AHS also added celecoxib oral solution to their consensus statement.

Celecoxib oral solution is currently the only available US Food and Drug Administration (FDA)–approved NSAID selectively inhibiting cyclooxygenase-2 (COX-2) for the acute treatment of migraine. It is the first therapeutic using SMEDDS technology to be recommended for acute migraine treatment, indicating acceptance of this drug delivery system [51]. The celecoxib oral solution SMEDDS formulation comprises 49.5% PEG-8 caprylic/capric glycerides (lipids with 8–10 carbons), a 40.5% mixture of the surfactants Tween 20 and propylene glycol monocaprylic ester (3:1), and 10% celecoxib [23]. The celecoxib oral solution SMEDDS formulation displays an increased rate of absorption and increased bioavailability, compared with the oral capsule formulation [52]. One study reported that the mean relative bioavailability of celecoxib oral solution is 140% greater than that of the celecoxib capsule, despite lower drug exposure (Fig. 1) [52]. The bioavailability in terms of the median time to maximum plasma concentration (AUC0–RefTmax) was 2.2-fold higher for the 120-mg dose of the celecoxib oral solution (1569 ± 299 ng·h/ml) than for the 400-mg oral capsule (777 ± 402 ng·h/ml) [52]. The Tmax for the celecoxib oral solution (0.7 h) was much shorter than that of the celecoxib capsule (2.5 h) [52]. At 1 h postdose, which is briefly after the Tmax of the celecoxib oral solution and within the time period a patient with migraine would expect pain relief, there was a pronounced difference in bioavailability. The area under the concentration–time curve (AUC0-1 h) of celecoxib oral solution vs. capsule was 605 ± 166 ng·h/ml and 103 ± 62 ng·h/ml, respectively [52].

Fig. 1
figure 1

Mean plasma concentration of celecoxib oral solution (DFN-15) compared with celecoxib capsules from A 0–72 h and B 0–3 h. This figure was modified from Pal et al. Clin Drug Investig. 2017 [52] under CC BY-NC 4.0 license terms [http://creativecommons.org/licenses/by-nc/4.0/]

Full size image

The improved PK profile of celecoxib oral solution likely underlies its early onset of action in the acute treatment of migraine, as observed in clinical trials. Two recent pivotal trials established the efficacy of celecoxib oral solution in acute migraine treatment, reflective of the rapid absorption provided by a SMEDDS formulation [53, 54]. Both studies achieved their coprimary endpoints of significantly more patients experiencing freedom from headache pain and freedom from most bothersome symptom at 2 h postdose [53, 54]. However, with a Tmax of 0.7 h, celecoxib oral solution delivered effective migraine pain relief as early as 1 h postdose to significantly more patients (Fig. 2) [52,53,54]. With the relatively low dose of celecoxib oral solution, the frequency of adverse events (AEs) was low and few AEs (e.g., dysgeusia and nausea) and no serious AEs were reported [53, 54]. These data show that optimized reformulation of drugs using SMEDDS technology can increase their speed of onset and clinical utility, despite lower overall drug exposure.

Fig. 2
figure 2

Proportion of patients from two clinical trials reporting headache pain relief at sequential time points after dosing of 120 mg of celecoxib oral solution. This figure was modified from Lipton et al. Headache 2020 [53] and Lipton et al. J Pain Res. 2021 [54] (excluding data from the outlier site) under CC BY-NC 4.0 license terms [http://creativecommons.org/licenses/by-nc/4.0/]

Full size image

A retrospective study of patients from the National Institutes of Diabetes and Digestive and Kidney Diseases Gastroparesis Clinical Research Consortium reported that 40.7% of patients diagnosed with gastroparesis also experience migraine [55]. Slow stomach emptying due to gastroparesis has been shown to delay absorption and reduce drug serum concentrations, which may lead to reduced efficacy of orally administered medications [56, 57]. The advantageous PK profile provided by SMEDDS technology in celecoxib oral solution may allow for increased absorption in the GI tract of patients with gastroparesis. The SMEDDS technology also facilitates the intestinal lymphatic absorption of celecoxib oral solution, which may contribute to the enhanced bioavailability as well.

Conclusions

SMEDDS technology increases both drug absorption and bioavailability of lipophilic drugs, compared with other formulations. This allows the use of lower doses with improved PK profiles without compromising efficacy. SMEDDS technology differs from SEDDS and SNEDDs mainly in thermodynamic stability, size, and surfactant/lipid composition. There has been increased usage of SMEDDS drug formulations in the clinic, including the recently FDA-approved celecoxib oral solution for the acute treatment of migraine. Celecoxib oral solution is a selective COX-2 inhibitor designed as an acute, low-dose, oral-solution SMEDDS formulation of celecoxib to treat migraine headaches acutely. It has an advantageous PK profile, allowing for a lower cumulative dose and a potentially lower risk of AEs than with non-SMEDDS celecoxib formulations. Celecoxib oral solution demonstrates the clinical utility of a SMEDDS formulation in the acute treatment of migraine.