Lipid based intramuscular long-acting injectables: current state of the art

Long acting injectables (LAI) have received increased research and commercial interest due to their potential for improving treatment effectiveness and adherence for antipsychotic, antiviral and addiction treatments. A range of materials have been used to formulate LAI products, including lipids and polymers. Classic lipid-based LAI, such as oil solutions of antipsychotic drugs, have been widely prescribed to patients. Clinical evidence has shown significantly improved key therapeutic markers such as reduction of relapses in the case of schizophrenia patients. The commercial LAI products can be given either via subcutaneous or intramuscular injection. The main types of lipid-based LAI formulations include oil solutions, lipid-based nanoparticles and lipid based liquid crystal formulations, which are currently clinically available, and oil suspensions and oleogels and which currently have no commercial products available. This review will discuss all relevant aspects related to the development of lipid-based long acting injectables with a special focus on intramuscular (IM) injectables. It aims to provide useful guidance on effective future LAI product design and development. Lipid-based nanoformulations are not discussed in this review as they are thoroughly reviewed in literature elsewhere.

See the excipients part in the study:

2.2. Excipients

2.2.1. Oils

Oily solution based LAI products are easy and simple to formulate, manufacture, sterilise and have good long-term stability (Weng Larsen and Larsen, 2009). Table 1 summarises the most commonly used oils in parenteral depot products. Unlike hydrogels, oils are particularly suitable for hydrolysis sensitive API’s. Most oils are generally known for their high biocompatibility upon injection (Lakshmi et al., 2017). Application of super-refined oils can lead to reduced oxidation and hydrolysis of the solubilised compounds (Motulsky et al., 2005). In general, oil solution based LAI products have a retention time limited to around four to six weeks (Weng Larsen and Larsen, 2009), but it is also drug-dependent. As an example,Nebidosolution for injectioncontaining testosterone undecanoateonly requires one injection of 750-1000 mg testosterone undecanoate administered every 10-14 weeks (Morgentaler et al., 2008).

2.2.2. Oleogelators

For oleogels, in addition to oil, gelators are needed to enable the formation of the network and the immobilisation of liquid oil. The gelators generally fall into two categories; low molecular weight oleogelators (LMOGs) (<1 kDa) and high molecular weight polymeric oleogelators (POGs) (>2 kDa) (Terech and Weiss, 1997). Low molecular weight gelators can gel an oil simply by direct dispersion,i.e.being dissolved in the oil by heating the mixture and then letting it crystallize upon cooling. Among the high molecular weight gelators, only ethylcellulose has enough solubility for the oil to form a gel by direct dispersion, but it is not biodegradablethus not ideal to be included (Patel, 2017). Structuring the oil with more hydrophilic biopolymers, such as proteins, is possible, but requires indirect methods as they are generally not dispersible in oil (de Vries et al., 2015). This class of gelators is not discussed here as they are out of the scope of this review.

In addition to their stability in water, one of the main problems of administering oleogels is their injectability, both with respect to the force required to inject them and their rheological destruction after passing through the needle. In situ forming oleogels are a promising tool to overcome this obstacle. Low molecular weight gelators have favourable properties for formulating in situ forming oleogels, includingthermoreversibility and,for some of them,their ability to self-assemble after the diffusion of a small amount of co-solvent. Despite the existence of a wide range ofmaterials that can be used as oleogelators, for developing LAI products, the biocompatibility and a good solubility in a biocompatible organic solvent of the material needs also to be taken into consideration.

Fatty acids

Medium to long chain fatty acids, such as stearic acid, palmitic acid, myristic acid, and arachidic acid are widely studied as gelators foroleogelsand are good candidates for pharmaceutical and food application due to their good biocompatibility. Preparation of these oleogels normally involve heating and melting of the fatty acid in oil followed by cooling which, causes crystallisation of the fatty acid and the formation of a solid network. Studies by Wang et al. (Wang et al., 2014) argued that in order to prevent tissue necrosis at the injection site, the ideal gelling temperature of these systems after the injection should be between body temperature (37°C) and 50°C. Using this temperature limitation, oleogels prepared using palmitic and myristic acid could not meet the standard. The oleogels containing stearic acid and arachidic acid at 7.5% and 5% concentration, respectively, can be prepared and then injected within this temperature range. The release of paliperidone from these oleogels were studied in vitro and compared to oil and aqueous suspensions. The oil suspension without any oleogelator extended the release to three days, compared to the 24 hours release of drug from the aqueous suspension. The addition of oleogelatorsto form oleogelsfurther prolonged the release to seven days, with a most sustained release achieved with 5% arachidic acid (Wang et al., 2014). The study also found that increasing the drug concentration also led to extension of drug release. The in vivo rodent data confirmed that the arachidic containing oleogels were able to form effective drug delivery depots post-injection and sustain the drug release over seven days governed by both diffusion and erosion mechanisms. After nine days the depothadcompletely disappeared (i.e.had beenfully absorbed) (Wang et al., 2014).

12-Hydroxy stearic Acid (12-HSA)

12-HSA is a LMOG that can form oleogels at very low concentrations (<1% w/w) in oils such as canola, peanut and soybean oils (M, 2017, Burkhardt et al., 2016, Rogers et al., 2009) as the continuous phase. As seen in Figure 3a, the phase diagram demonstrates that 12-HSA can form gels with canola oil at low temperatures when a critical concentration of 0.5% w/w is reached. The strength and appearance of the gels are 12-HSA concentration and temperature dependent. In addition, external factors such as shear stress and cooling rate during the preparation of the 12-HSA containing oleogelscan also affect the microstructures of the gels. Co and Marangoni studied the combined effects of applied laminar oscillatory shear and cooling rate applied during crystallization on the microstructures of 12-HSA/canola oil oleogels (Rogers et al., 2009, Co and Marangoni, 2013) . They reported that crystallization at a high cooling rate (30 ºC/min) resulted in a spherulitic microstructure (Figure 3b) with a higher oil-binding capacity, lower storage modulus and lower yield stress compared with a material (with a fibrillar microstructure) crystallized at a slow cooling rate (1 ºC/min) (Co and Marangoni, 2013). As seen in Figure 3b, increasing the frequency of the oscillatory shear applied increased the incidence of nucleation for the rapidly cooled gel, but had no effect on the slow cooled gel (Co and Marangoni, 2013). With sufficiently high frequency (i.e. 10.0 Hz, Figure 3b, iv) of oscillation, the spherulisation (fibre branching) and growth was inhibited, but the mechanism of this observation is unclear (Co and Marangoni, 2013).

Lipid based intramuscular long-acting injectables: current state of the art

Figure 3. (a)The phase diagram of gelation behaviour when canola oil and 12-HSA are combined. Rogerset al., 2009 (Rogers et al., 2009) with permission; (b)The microstructure of 12-HSA/canola oleogels cooled at 30 ºC/min and shear-crystallized (oscillatory) under various frequencies (strain = 1,500 %): 2.5 Hz (i), 5.0 Hz (ii), 7.5 Hz (iii), 10.0 Hz (iv). Co and Marangoni, 2013 (Co and Marangoni, 2013) with permission.


To formulate in situ forming oleogels, 12-HSA was used with solvent/gelation inhibitors including 2-pyrrolidone, ethyl acetate, NMP, DMSO, glycofurol, and PEG 400 and NMP was selected as optimal for its good oil miscibility and high solvating power for 12-HSA (M, 2017) .Tantishaiyakulet al. reported that 4% ethanol inhibited gelation of 12-HSA in coconut oil and the formulation could be easily injected through a 21-gauge needle. The in vitro drug release of the gels containing of the model lipophilic and hydrophilic drugs, piroxicam and diclofenac acid, were slower when compared to oily suspensions (Tantishaiyakul et al., 2018). For delivering testosterone enanthate and leuprolide acetate using in situ forming oleogels, the addition of no more than 15% of NMP was sufficient to completely inhibit gelation of peanut oil-based formulations containing 5 % and 7 % of 12-HSA and form solutions which were stable for 6 months and injectable through 26-gauge needles (M, 2017). However, the authors highlighted that convincing data interpretation and prediction of the in vivo drug release of the oleogels was difficult. This is not only due to the small sample sizes of the in vivo data, but also due multiple factors relating to the complex phase transformation (in situ solidification upon NMP extraction which also was believed to be at least partially responsible for the observed initial burst release) of the formulation after injections. Post injection, the surface erosion, fragmentation and deformation of the implants may also contribute to the unpredictability of the measured in vivo behaviour of the formulation (M, 2017).

In its native form, due to its free carboxylic group, 12-HSA is suggested to cause skin irritation, thuswork has been carried out to modify 12-HSA by ethoxylation in order to reduce the toxic effects. Berkhardt and co-workers (Burkhardt et al., 2016) reported that ethoxylation of 12-HSA provided an structure capable of gelling a variety of solvents, including decamethylenecyclopentasiloxane, 2-ethylhexyl palmitate and bis(2-ethylhexylcarbonate), and oils (paraffin and caprinic/caprylic triglyceride). Interestingly, 12-HSA with the highest degree of ethoxlaytion provided a gelator with the most potent gelling capability (Burkhardt et al., 2016).

Amino acid derivates

Aminoacid based gelators are the most studied for biomedical applications for their intrinsic biocompatibility. Fatty acid derivatives of aminoacids have also proved to selectively gel oil in an aqueous environment, making them optimal candidates as oleogels for injection in human body.Amphiphilic derivatives of L-alanine have been shown to form oleogels when combined with oils and in the presence of water (Couffin-Hoarau et al., 2004, Bastiat and Leroux, 2009, Motulsky et al., 2005, Bastiat et al., 2010). N-Stearoyl-l-alanine methyl ester (SAM) and N-stearoyl L-alanine ethyl ester (SAE) were used as gelators to produce gelled water-in-oil emulsions loaded with leuprolide. They were produced with safflower oil as the continuous phase and NMP as a gelation inhibitor (Figure 4). Characterisation of the depots by differential scanning calorimetry (DSC) and texture analysis showed that the 10% SAM system exhibited a higher gel–sol phase transition temperature than its SAE counterpart derivative and a 30% increase in strength and retained leuprolide for a longer period (24.0 ± 1.4 (SAM) versus 34.4 ± 5.9% (SAE) released after 72 hours), as demonstrated in Figure 4b (Plourde et al., 2005). It was suggested that the difference may be related to fast gelation kinetics which led tothe low porosity and high stiffness of the formed gel. The greater stiffness of SAM gels was attributed to a higher level of H-bonding between the amide functions of the oleogelator, which, in case of SAE, was partially hindered by the ethyl group. For this reason, the most promising derivativefor forming LAIs in the L-alanine class seems to be SAM. Oleogels prepared with SAM show greater drug retention properties than hydrogels and comparable release profile to polymeric microspheres (Plourde et al., 2005).

Lipid based intramuscular long-acting injectables: current state of the art

Figure 4. (a)Schematic of the preparation of a safflower and L-alanine oleogel to deliver Leuprolideand (b) the kinetics of release. Plourdeet al. 2005 (Plourde et al., 2005) with permission.

SAM was used to gel sunflower oil containing rivastigmine or rivastigmine hydrogen tartrate ina dissolved or dispersed state respectively (Vintiloiu et al., 2008). The in vitro data showed that rivastigmine oil suspension hada considerably smaller burst drug release effect in comparison to the oil solution (Vintiloiu et al., 2008). The pharmacokinetics results of SC injected suspension to rats showed no statistical difference in values of area under the rivastigmine blood concentration versus time curve (AUC) for 14 days between oil based and oleogel formulations. Nevertheless, the burst release decreased with increasing gelator concentration, contrary to in vitro studies. Decreasing the burst release could allow the incorporation of a higher amount of drug, thus prolonging the release. The authors hypothesised that higher SAM oleogelator concentration led to the higher density of network. This improved cohesiveness of these gels, which may reduce the subcutaneous spreading of the formulation after injection. Minimal spreading gave apotentially small surface area which generated minimized drug contact with the aqueous environment, contributing to a decrease in burst release (Vintiloiu et al., 2008).

Work has also been carried out on the in vitro and in vivo degradation of SAM oleogels combined with soybean oil and NMP (Wang et al., 2010). A correlation between the concentration of SAM in the oleogel and the time it took to degrade in PBS (8%, 10%, 12%, and 15% SAM oleogels degraded by 35.40%, 31.32%, 25.39%, and 17.50% over 40 days) was reported suggesting that a dense gel could block water molecules from entering the 3D network. An in vivo study demonstrated that the 12% SAM oleogel was biodegradable with very little remaining after six-weeks, where a gel weight loss of 94.57% was observed, which was significantly faster than the in vitro results (25.39%). The distinct difference between the in vivo and in vitromethods of evaluation may raise questions on the direct correlations between in vivo and in vitro data for both biodegradability and drug release. In terms of toxicity, MTT cell viability studies of the SAM oleogels using mouse fibroblasts (L929) showed some evidence of toxicity when the gel extracts were diluted 1:5. When diluted further to 1:20, no toxicity was detected (Wang et al., 2014).

Despite the promising results, L-alanine based gelators assemble through relatively weak molecular interactions, leading to the formation of gels with poor mechanical properties and require a high concentration of gelator. Tyrosine based gelators, in particular the N-behenoyl L-tyrosine methyl ester (BTM), has shown to have better gelation ability, rheological properties and drug release profiles compared to L-alanine based gelators in vitro (Bastiat and Leroux, 2009). Animal studies conducted in rats concluded that both SAM and BTM were able to prolong the release of rivastigmine hydrogen tartrate compared to the oil suspension, from 2 to 10 days (Bastiat et al., 2010). The gelation of the oil greatly decreased the burst release, allowing the administration of a greateramount of drug. This effect was two times greaterfor BTM compared to SAM.

Methyl (S)-2,5-ditetradecanamidopentanoate (MDP) was synthetized as a new oleogelator to improve the mechanical properties of oleogels formed with L-amino acid derivatives (Li et al., 2016). The amounts of MDP necessary to structure oils, such as injectable soybean oil, olive oil and corn oil,werebelow 5% (w/v) whichwas much less compared to SAM in which the gelator concentration was between 7.5 to 10 % (w/v) in safflower oil. This improvement was attributed to the more H-bonding sites, s as a result of the greater number of amide groups. The release of candesartan cilexetil was investigated in vitro as to optimize the amount of gelator, drug and gelator inhibitor. The chosen formulation contained 4.9% of MDP and 30% of NMP and was tested in vivo and compared to an oil solution. The oleogel formulation significantly prolonged the release and hadgood biocompatibility.

2.2.3. Solvents

For LAIs, solvents are used in in situ forming dosage forms, such as oleogels and LCFSs. Solvents reduce the viscosity of the formulationby inhibiting crystallisation and preventinglipid solidification and ensure the formulation is injectable. Using NMP containing oleogels as examples, NMP will partially disrupt the interactions between the oleogelator molecules, thereby maintaining the formulation in a liquid state and hence ease the injection. Once in situ, the solvent diffuses into the surrounding tissue, allowing the oil and gelator to interact and form a solid depot (Hatefi and Amsden, 2002, Plourde et al., 2005, M, 2017).The diffusion of the solvent may, in some cases, led to burst drug release if the drug has a high solubility in the specific solvent, thus the optimisation of the quantity used is required. The commonly used organic solvents in oleogels and LCFSs are NMP, DMSO, ethanol (Vintiloiu et al., 2008, Plourde et al., 2005, Tantishaiyakul et al., 2018, Couffin-Hoarau et al., 2004, M, 2017, Wang et al., 2010).

The use of solvents in IM injectable formulations is, however, controversial. Some studies suggested that the solvent may cause potential muscle damage at the injection site. Kranz et al. (Kranz et al., 2001) evaluated muscle damage in vivo by measuring the cumulative creatine kinase (CK) release from the musclesafter injection with either the pure solvent, including NMP, DMSO and 2-pyrrolidone or in situ forming depots containing the same solvent. Their results showed a significant increase in CK when the solvent was injected alone in comparison to a control injection of saline. 2-pyrrolidone demonstrated the lowest toxicity in comparison to other solvents investigated (Kranz et al., 2001). In situ forming depotsolutions containing 40% PLA, and the same solvents discussed above, showed comparable CK-efflux to the pure solvents. However, when peanut oil was added to the formulation the CK-levels were significantly lower and decreased with increasing amount of external oil phase. The authors hypothesized that the external oil in the in situ forming formulations acted as a barrier and prevented the immediate contact of the solvent with the muscle (Kranz et al., 2001). There are other studies indicatingthat the irritation caused by the use of solvent in the IM injectables is minimal when using inflammation as a marker. For example, Motlusky and colleagues used NMP in an oleogelcomposed of L-alanine and safflower oil (Motulsky et al., 2005). The biocompatibility study showed a minimal inflammatory reaction which was limited to the immediate area around the geland within the range of expected foreign body reaction. In comparison to polymeric microsphere based LAIs, the significantly lower surface area/volume may also play a role in the low tissue inflammation observed (Anderson and Shive, 1997).

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Janine Wilkinson, Damilola Ajulo, Valeria Tamburrini, Gwenaelle Le Gall, Kristof Kimpe, Rene Holm, Peter Belton, Sheng Qi, Lipid based intramuscular long-acting injectables: current state of the art, European Journal of Pharmaceutical Sciences, 2022, 106253, ISSN 0928-0987,

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