madman
Super Moderator
Abstract
Oil depots are parenteral drug formulations meant for the sustained release of lipophilic compounds. According to mass transport models, the drug-release rate from these injections is determined by the surface area of the oil depot. Until now, the size of the surface area of injected depots has not been assessed, however. MRI provides an excellent possibility to distinguish between water and adipose tissue. The aim of this study was to investigate whether MRI can be used to determine the shape and hence the surface area of oil depots in muscle tissue. The developed MRI-scan protocol is demonstrated to be suitable for visualizing oil depots. It was applied to determine the surface area of 0.5 mL oil, i.m. injected in healthy volunteers. The mean (± RSD) surface area and volume of the depots recovered after injection was 755.4 mm2 (± 26.5) and 520.1 mm3 (± 24.6). It is shown that the depot disappearance from the injection site is very variable between volunteers. It is suggested that the oil is first solubilized and subsequently distributed. In all cases, the oil was not detectable after 14 days. These factors are relevant for the understanding of the mechanism by which compounds are released out of oil depots.
1. Introduction
Sustained delivery of drugs is an important method of drug administration for a number of diseases that require long-term drug treatment, such as psychiatric disorders (Covell et al., 2012; Novakovic et al., 2013; Uchida et al., 2013; Van Weringh et al., 1994) and hormone-dependent conditions (Edelstein and Basaria, 2010; Geusens, 1995; Morgentaler et al., 2008). The most commonly used drug-delivery systems, which can release drugs for a longer period of time, are parenteral injections. In general, long-acting parenteral injections are mostly administered by the intramuscular (i.m.) and subcutaneous (s.c.) route (Prettyman, 2005).
A considerable number of long-acting i.m. injections are available on the market. Conventional sustained-release injections often consist of lipophilic compounds dissolved in vegetable oils. For example, Arachis oil depots containing nandrolone decanoate (Bagchus et al., 2005; Minto et al., 1997; Wijnand et al., 1985) and castor oil depots containing testosterone undecanoate (Morgentaler et al., 2008) or estradiol valerate (Düsterberg and Nishino, 1982) have been marketed several decades ago and are still available on the market. Furthermore, i.m. oil depots are widely used as parenteral depot formulations of antipsychotic drugs, such as haloperidol decanoate (Van Weringh et al., 1994). Since these long-acting formulations are administered every few weeks, they require fewer injections and result in improved drug compliance.
Although many i.m. oil depots for sustained drug delivery have been marketed, the rate and extent of drug release are often difficult to predict. The drug-release and absorption rate from the oil solution is controlled by the drug partitioning between the oil vehicle and the tissue fluid (Kalicharan et al., 2016b). However, several other factors such as the injection site (Minto et al., 1997; Shaik et al., 2015; Soni et al., 1988), injection volume (Minto et al., 1997), the rate of bioconversion of the prodrug into the parent drug, the absorption and distribution of the oil vehicle and the extent of spreading of the depot at the injection site might affect the overall pharmacokinetic profile of the drug (Larsen et al., 2009; Weng Larsen and Larsen, 2009). Most studies focus on the pharmacokinetics of the drug rather than on the fate of the oil depot formulation (Bagchus et al., 2005; Luo et al., 1997; Morgentaler et al., 2008; Soni et al., 1988; Van Weringh et al., 1994; Wijnand et al., 1985). Thus far, only a few researchers have reported on the rate and extent of the disappearance of the injected formulation. In 2001 Larsen et al. reported a disappearance half-life of 21.4 days of an iodine-125 labeled oil depot after i.m. injection in the lower back of pigs (Larsen et al., 2001). Radioactivity was monitored by placing the scintillator probe directly on the skin surface. To date, no studies on the fate of oil depots in humans have been published yet.
The drug-release rate from depot injections is estimated according to mass transport models. A parameter for this release rate is the surface area of the oil depot (Nelson and Shah, 1975). These mass transport models are often based on the assumption that the injected depots are spherically shaped. However, until now, the shape, and therefore the associated surface area, of injected depots had not been known. The surface area is the interface between a hydrophilic and a lipophilic phase. In the case of an i.m. oil depot, the interface is formed by the muscle interstitial fluid and the oil formulation, respectively. Since the size of the surface area of the oil depot determines the extent of mass transport over the interface, this plays an important role in the rate and extent of drug release from i.m. injected oil depots (Nelson and Shah, 1975).
Until now, there are no published methods to visualize small volumes of oil in situ without an invasive procedure. Hence, the value of the surface area of a perfect sphere is used for modeling the release rate of substances. As a result, absorption profiles of active substances originating from oil depots cannot be accurately predicted and the pharmacokinetic profiles and therapeutic efficacy cannot be estimated.
In this study, magnetic resonance imaging (MRI) was used to determine the surface area of oil depots in a non-invasive manner. MRI provides excellent soft-tissue contrasts and this feature can also be used to distinguish water and adipose (fat) tissue. The fat/ water contrast in images can be provoked in two different ways: (1) by signal weighting on the basis of the magnetic relaxation times (T1 and T2 for the longitudinal and transverse magnetization component, respectively) of protons in the oil depot and of those in the surrounding tissue or (2) by the difference in chemical shift (Philips Medical Systems, 2008). Each nuclei has a different spin frequency and this frequency is also influenced by nearby nuclei (e.g. in a molecular environment by chemical bonding). Chemical shifts are relative frequency differences within one voxel. Initially, both imaging techniques were used in preliminary studies. Later on, the focus was shifted to the difference in chemical shift, because this technique is generally known for obtaining fat fractions (small portions of lipophilic liquid or tissue) adequately in a quantitative result. This is relevant in this current study.
The aim of this study was to investigate whether MRI can be used to determine the surface area of oil depots in muscle tissue in situ. Subsequently, the developed MRI-scan protocol was applied to determine the surface area of i.m. injected oil depots in human volunteers.
3. Results and Discussion
This study reports the development of an MRI method to visualize the surface area of an i.m. oil depot injection. Firstly, preliminary studies were conducted in order to develop an appropriate MRI-scan protocol. Secondly, the developed scan protocol was assessed on suitability for use in humans. Lastly, the MRI-scan protocol was used to determine the surface area of i.m. injected oil depots in healthy volunteers.
3.2. Clinical study
To study the surface shape of an oil depot in humans, four healthy male volunteers were injected with an oil depot in the biceps brachii of the left arm. The baseline characteristics of the volunteers are summarized in Table 1. The study was designed to scan each volunteer four times in total. This number was requested to keep the burden for the volunteer as low as possible, while we were still able to obtain the rate of disappearance from the injection site (zero- or first-order clearance kinetics).
The mean (RSD) surface area and volume of the depots recovered after injection was 755.4 mm2 (26.5) and 520.1 mm3 (24.6). All oil depots were injected between 15 and 22 mm deep. No adverse reactions upon injection were reported.
As can be seen in Fig. 4, the injected depots were oblong in all volunteers. This stretched shape was confirmed by post-processing the data (Fig. 5). Apparently, the oil liquid follows the fibers in the muscle. A schematic cross-section of the muscle before and after injection is represented in Fig. 6. The resolution of the MRI scanner was not sufficient enough to visualize perimysia and fascicles. Therefore, it cannot be determined how the oil liquid is spread through the perimysium and several fascicles. However, based on the size of the needle, it is assumed that the perimysium between fascicles will be pierced by the injection needle. Consequently, as a result of the injection, the oil liquid will spread across multiple fascicles and will form a bulk (continuous) phase (Fig. 6). This is substantiated by the visualization of the oil depots immediately after injection (Fig. 5), wherein the oil depot forms a thin, long shape.
3.2.1. Disappearance rate from the injection site
In addition to the visualization of the shape of the oil depot, the rate of disappearance of the oil depot from the injection site was also determined. As stated before, the reported disappearance half-life of an i.m. depot in pigs was 21.4 days (Larsen et al., 2001). Based on these data, it was estimated that the depot would stay for 4–5 weeks at the injection site. Therefore, a total of 4 MRI-scans were scheduled for every volunteer in the consecutive weeks after injection.
Volunteer A and B received the first oil injections and scanned according to the scan schedule as mentioned in the study protocol. In the second week after injection, the oil depot volume in volunteer A decreased from 506.0 to 178.6 mm3; this was a reduction of 327.4 mm3 (65%) within 8 days (Fig. 7A). Furthermore, the depot could not be visualized anymore on day 14. Speculatively, a merge with adipose tissue occurred or the oil depot was fully stretched out, probably due to oil digestion. See an evaluation of the applied method below.
The second MRI-scan for volunteer B was, as scheduled, in the second week after injection. The depot was not visible anymore at that moment (Fig. 7). Hence, it was unclear when the depot disappeared exactly, but it was within 2 weeks.
The scan schedule for volunteer C and D was therefore intervened. The period between the MRI-scans for volunteers C and D was shortened. The oil depot in volunteer C was determined at days 0, 4, 8, and 14 (Fig. 7). Clearly, the oil depot was split into 2 or more parts (Fig. 8).
The oil depot in volunteer C still showed a fast disappearance rate from the injection site (Fig. 7). Therefore, it was decided to modify the scan intervals for volunteer D by scanning the upper arm every day with a maximum of 4 scans. The depot volume was 540.8 mm3 immediately after injection. The volume was reduced by 430 mm3 after the first day. A full disappearance was seen on day 2. This volunteer had normal motility during these days. There was neither sports activity nor heavy objects were lifted.
3.3. Evaluation of applied developed method in the current clinical study
The here developed MRI scan method is accurate, precise, and sensitive and it is therefore applied in the current clinical study. An arbitrary cut-off voxel intensity value of 0.5 was used to label fat fractions. Fig. 9 shows two other evaluated cut-off voxel intensity values. Logically, the surface area and volume increase by a cut-off voxel intensity value of 0.3 for the fat fraction, whereas these oil depot properties decrease when using 0.7. Interestingly, the 3D image shows a smaller oil depot by a cut-off voxel value of 0.7. It seemed that the whole oil depot contained less fat fractions by, speculatively, an increased amount of water per voxel. This may result in a fine dispersion of small oil droplets emulsified in interstitial fluid. These droplets are not anymore attached to the main part of the oil depot. It can be speculated that these small droplets are individually cleared from the injection site. Although there is no clearance or digestion mechanism published yet, it can be suggested that these droplets can be transported towards the lymphatics or stay attached to the muscle fibers (Fig. 6). As a result of this speculation, these small droplets have a relatively high surface area that may cause a quicker oil depot depletion of compounds.
4. Conclusion
The MRI method applied in this study is able to visualize the shape of an oil depot when injected into the muscle. The method enables one to estimate the surface area as well as the way the oil is disappeared from the injection site. From this, a further understanding can be obtained about the mechanism of drug absorption from oil depots. During the development of an i.m. injection, it should be taken into account that a stretched shape of the oil depot is formed in muscle. It can be argued that the oil depot is squeezed between the muscle fibers, which explains the obtained shape. As a result of this shape, the determined surface area is much larger than that of a perfect sphere that is used in mathematical models. Although the surface areas were approximately the same in all volunteers directly after injection, this was not the case in the successive days. Furthermore, the oil depot disappearance from the injection site is very variable between patients after i.m. injection. In all cases, the oil depot was disappeared from the injection site within 14 days. These factors are relevant for the absorption kinetics of active substances from oil depots and therefore contribute to the optimal therapeutic treatment in patients.
Oil depots are parenteral drug formulations meant for the sustained release of lipophilic compounds. According to mass transport models, the drug-release rate from these injections is determined by the surface area of the oil depot. Until now, the size of the surface area of injected depots has not been assessed, however. MRI provides an excellent possibility to distinguish between water and adipose tissue. The aim of this study was to investigate whether MRI can be used to determine the shape and hence the surface area of oil depots in muscle tissue. The developed MRI-scan protocol is demonstrated to be suitable for visualizing oil depots. It was applied to determine the surface area of 0.5 mL oil, i.m. injected in healthy volunteers. The mean (± RSD) surface area and volume of the depots recovered after injection was 755.4 mm2 (± 26.5) and 520.1 mm3 (± 24.6). It is shown that the depot disappearance from the injection site is very variable between volunteers. It is suggested that the oil is first solubilized and subsequently distributed. In all cases, the oil was not detectable after 14 days. These factors are relevant for the understanding of the mechanism by which compounds are released out of oil depots.
1. Introduction
Sustained delivery of drugs is an important method of drug administration for a number of diseases that require long-term drug treatment, such as psychiatric disorders (Covell et al., 2012; Novakovic et al., 2013; Uchida et al., 2013; Van Weringh et al., 1994) and hormone-dependent conditions (Edelstein and Basaria, 2010; Geusens, 1995; Morgentaler et al., 2008). The most commonly used drug-delivery systems, which can release drugs for a longer period of time, are parenteral injections. In general, long-acting parenteral injections are mostly administered by the intramuscular (i.m.) and subcutaneous (s.c.) route (Prettyman, 2005).
A considerable number of long-acting i.m. injections are available on the market. Conventional sustained-release injections often consist of lipophilic compounds dissolved in vegetable oils. For example, Arachis oil depots containing nandrolone decanoate (Bagchus et al., 2005; Minto et al., 1997; Wijnand et al., 1985) and castor oil depots containing testosterone undecanoate (Morgentaler et al., 2008) or estradiol valerate (Düsterberg and Nishino, 1982) have been marketed several decades ago and are still available on the market. Furthermore, i.m. oil depots are widely used as parenteral depot formulations of antipsychotic drugs, such as haloperidol decanoate (Van Weringh et al., 1994). Since these long-acting formulations are administered every few weeks, they require fewer injections and result in improved drug compliance.
Although many i.m. oil depots for sustained drug delivery have been marketed, the rate and extent of drug release are often difficult to predict. The drug-release and absorption rate from the oil solution is controlled by the drug partitioning between the oil vehicle and the tissue fluid (Kalicharan et al., 2016b). However, several other factors such as the injection site (Minto et al., 1997; Shaik et al., 2015; Soni et al., 1988), injection volume (Minto et al., 1997), the rate of bioconversion of the prodrug into the parent drug, the absorption and distribution of the oil vehicle and the extent of spreading of the depot at the injection site might affect the overall pharmacokinetic profile of the drug (Larsen et al., 2009; Weng Larsen and Larsen, 2009). Most studies focus on the pharmacokinetics of the drug rather than on the fate of the oil depot formulation (Bagchus et al., 2005; Luo et al., 1997; Morgentaler et al., 2008; Soni et al., 1988; Van Weringh et al., 1994; Wijnand et al., 1985). Thus far, only a few researchers have reported on the rate and extent of the disappearance of the injected formulation. In 2001 Larsen et al. reported a disappearance half-life of 21.4 days of an iodine-125 labeled oil depot after i.m. injection in the lower back of pigs (Larsen et al., 2001). Radioactivity was monitored by placing the scintillator probe directly on the skin surface. To date, no studies on the fate of oil depots in humans have been published yet.
The drug-release rate from depot injections is estimated according to mass transport models. A parameter for this release rate is the surface area of the oil depot (Nelson and Shah, 1975). These mass transport models are often based on the assumption that the injected depots are spherically shaped. However, until now, the shape, and therefore the associated surface area, of injected depots had not been known. The surface area is the interface between a hydrophilic and a lipophilic phase. In the case of an i.m. oil depot, the interface is formed by the muscle interstitial fluid and the oil formulation, respectively. Since the size of the surface area of the oil depot determines the extent of mass transport over the interface, this plays an important role in the rate and extent of drug release from i.m. injected oil depots (Nelson and Shah, 1975).
Until now, there are no published methods to visualize small volumes of oil in situ without an invasive procedure. Hence, the value of the surface area of a perfect sphere is used for modeling the release rate of substances. As a result, absorption profiles of active substances originating from oil depots cannot be accurately predicted and the pharmacokinetic profiles and therapeutic efficacy cannot be estimated.
In this study, magnetic resonance imaging (MRI) was used to determine the surface area of oil depots in a non-invasive manner. MRI provides excellent soft-tissue contrasts and this feature can also be used to distinguish water and adipose (fat) tissue. The fat/ water contrast in images can be provoked in two different ways: (1) by signal weighting on the basis of the magnetic relaxation times (T1 and T2 for the longitudinal and transverse magnetization component, respectively) of protons in the oil depot and of those in the surrounding tissue or (2) by the difference in chemical shift (Philips Medical Systems, 2008). Each nuclei has a different spin frequency and this frequency is also influenced by nearby nuclei (e.g. in a molecular environment by chemical bonding). Chemical shifts are relative frequency differences within one voxel. Initially, both imaging techniques were used in preliminary studies. Later on, the focus was shifted to the difference in chemical shift, because this technique is generally known for obtaining fat fractions (small portions of lipophilic liquid or tissue) adequately in a quantitative result. This is relevant in this current study.
The aim of this study was to investigate whether MRI can be used to determine the surface area of oil depots in muscle tissue in situ. Subsequently, the developed MRI-scan protocol was applied to determine the surface area of i.m. injected oil depots in human volunteers.
3. Results and Discussion
This study reports the development of an MRI method to visualize the surface area of an i.m. oil depot injection. Firstly, preliminary studies were conducted in order to develop an appropriate MRI-scan protocol. Secondly, the developed scan protocol was assessed on suitability for use in humans. Lastly, the MRI-scan protocol was used to determine the surface area of i.m. injected oil depots in healthy volunteers.
3.2. Clinical study
To study the surface shape of an oil depot in humans, four healthy male volunteers were injected with an oil depot in the biceps brachii of the left arm. The baseline characteristics of the volunteers are summarized in Table 1. The study was designed to scan each volunteer four times in total. This number was requested to keep the burden for the volunteer as low as possible, while we were still able to obtain the rate of disappearance from the injection site (zero- or first-order clearance kinetics).
The mean (RSD) surface area and volume of the depots recovered after injection was 755.4 mm2 (26.5) and 520.1 mm3 (24.6). All oil depots were injected between 15 and 22 mm deep. No adverse reactions upon injection were reported.
As can be seen in Fig. 4, the injected depots were oblong in all volunteers. This stretched shape was confirmed by post-processing the data (Fig. 5). Apparently, the oil liquid follows the fibers in the muscle. A schematic cross-section of the muscle before and after injection is represented in Fig. 6. The resolution of the MRI scanner was not sufficient enough to visualize perimysia and fascicles. Therefore, it cannot be determined how the oil liquid is spread through the perimysium and several fascicles. However, based on the size of the needle, it is assumed that the perimysium between fascicles will be pierced by the injection needle. Consequently, as a result of the injection, the oil liquid will spread across multiple fascicles and will form a bulk (continuous) phase (Fig. 6). This is substantiated by the visualization of the oil depots immediately after injection (Fig. 5), wherein the oil depot forms a thin, long shape.
3.2.1. Disappearance rate from the injection site
In addition to the visualization of the shape of the oil depot, the rate of disappearance of the oil depot from the injection site was also determined. As stated before, the reported disappearance half-life of an i.m. depot in pigs was 21.4 days (Larsen et al., 2001). Based on these data, it was estimated that the depot would stay for 4–5 weeks at the injection site. Therefore, a total of 4 MRI-scans were scheduled for every volunteer in the consecutive weeks after injection.
Volunteer A and B received the first oil injections and scanned according to the scan schedule as mentioned in the study protocol. In the second week after injection, the oil depot volume in volunteer A decreased from 506.0 to 178.6 mm3; this was a reduction of 327.4 mm3 (65%) within 8 days (Fig. 7A). Furthermore, the depot could not be visualized anymore on day 14. Speculatively, a merge with adipose tissue occurred or the oil depot was fully stretched out, probably due to oil digestion. See an evaluation of the applied method below.
The second MRI-scan for volunteer B was, as scheduled, in the second week after injection. The depot was not visible anymore at that moment (Fig. 7). Hence, it was unclear when the depot disappeared exactly, but it was within 2 weeks.
The scan schedule for volunteer C and D was therefore intervened. The period between the MRI-scans for volunteers C and D was shortened. The oil depot in volunteer C was determined at days 0, 4, 8, and 14 (Fig. 7). Clearly, the oil depot was split into 2 or more parts (Fig. 8).
The oil depot in volunteer C still showed a fast disappearance rate from the injection site (Fig. 7). Therefore, it was decided to modify the scan intervals for volunteer D by scanning the upper arm every day with a maximum of 4 scans. The depot volume was 540.8 mm3 immediately after injection. The volume was reduced by 430 mm3 after the first day. A full disappearance was seen on day 2. This volunteer had normal motility during these days. There was neither sports activity nor heavy objects were lifted.
3.3. Evaluation of applied developed method in the current clinical study
The here developed MRI scan method is accurate, precise, and sensitive and it is therefore applied in the current clinical study. An arbitrary cut-off voxel intensity value of 0.5 was used to label fat fractions. Fig. 9 shows two other evaluated cut-off voxel intensity values. Logically, the surface area and volume increase by a cut-off voxel intensity value of 0.3 for the fat fraction, whereas these oil depot properties decrease when using 0.7. Interestingly, the 3D image shows a smaller oil depot by a cut-off voxel value of 0.7. It seemed that the whole oil depot contained less fat fractions by, speculatively, an increased amount of water per voxel. This may result in a fine dispersion of small oil droplets emulsified in interstitial fluid. These droplets are not anymore attached to the main part of the oil depot. It can be speculated that these small droplets are individually cleared from the injection site. Although there is no clearance or digestion mechanism published yet, it can be suggested that these droplets can be transported towards the lymphatics or stay attached to the muscle fibers (Fig. 6). As a result of this speculation, these small droplets have a relatively high surface area that may cause a quicker oil depot depletion of compounds.
4. Conclusion
The MRI method applied in this study is able to visualize the shape of an oil depot when injected into the muscle. The method enables one to estimate the surface area as well as the way the oil is disappeared from the injection site. From this, a further understanding can be obtained about the mechanism of drug absorption from oil depots. During the development of an i.m. injection, it should be taken into account that a stretched shape of the oil depot is formed in muscle. It can be argued that the oil depot is squeezed between the muscle fibers, which explains the obtained shape. As a result of this shape, the determined surface area is much larger than that of a perfect sphere that is used in mathematical models. Although the surface areas were approximately the same in all volunteers directly after injection, this was not the case in the successive days. Furthermore, the oil depot disappearance from the injection site is very variable between patients after i.m. injection. In all cases, the oil depot was disappeared from the injection site within 14 days. These factors are relevant for the absorption kinetics of active substances from oil depots and therefore contribute to the optimal therapeutic treatment in patients.
Last edited:
