madman
Super Moderator
Abstract
Context: There is no licensed oral Native Testosterone (NT) because of challenges in the formulation. Licensed oral formulations of the ester, testosterone undecanoate (TU), require a meal for absorption and generate supraphysiological dihydrotestosterone (DHT) levels.
Objective: To develop an oral NT formulation.
Design and Methods: A lipid-based formulation of native testosterone-filled into soft-gelatin capsules at 40mg per capsule was designed with 2 years of stability at ambient temperature. Pharmacokinetic comparison studies of this oral lipidic NT formulation to oral TU were conducted in dogs and hypogonadal men.
Results: In dogs, 40mg NT was well absorbed under fasted conditions whereas 40mg TU required a high-fat meal: for NT the mean fed/fasted AUC ratio was 1.63 and for TU 7.05. In hypogonadal men fed and fasted NT had similar pharmacokinetics: Cmax mean 26.5 vs 30.4 nmol/l (769 vs 882 ng/dl), AUC0-10h 87 vs 88.6 h*nmol/l. NT (fed state) showed a testosterone AUC increase of 45% between 120mg and 200mg and NT 200mg gave a similar mean AUC0-10h to TU 80mg: 87 vs. 64.8 h*nmol/l. Serum TU levels were variable and on a molar basis were ~10-fold higher than serum testosterone levels after TU 80mg fed. The DHT: testosterone AUC0-10h ratio was more physiological for NT than TU being 0.19 vs 0.36. There were no emerging safety concerns with NT.
Conclusion: This novel oral lipidic native testosterone formulation has potential advantages over oral TU of dosing independently of food and a lower risk of supraphysiological DHT levels.
Significance Statement
There is no licensed oral testosterone because of challenges in formulation, and the oral formulations of the ester, testosterone undecanoate, require a fatty meal for absorption and generate supraphysiological dihydrotestosterone levels. We have overcome the design challenges and formulated oral native testosterone which can be taken with or without food and provides physiological levels of testosterone and dihydrotestosterone in hypogonadal men. This formulation, DITEST, has the potential advantage of being oral for patients who don’t tolerate injections and less risk of adverse events that might theoretically be associated with elevated dihydrotestosterone levels. Future studies will need to define the dosing regimen for replacement in hypogonadal men.
Introduction
Testosterone was isolated, named, and synthesized in 1935 (1), but to date, no oral native testosterone has been licensed for testosterone replacement therapy. The reason being that oral native testosterone, although absorbed through the intestine, undergoes extensive presystemic metabolism along the gastrointestinal tract (2), as well as rapid first-pass metabolism in the liver (3). The oral absorption of testosterone is also dependent on the dosing vehicle, wherein a lipophilic vehicle may increase the proportion of testosterone absorbed via the lymphatic route (4). It is thus difficult to achieve adequate bioavailability of testosterone in order to maintain consistent physiological testosterone levels via the oral route. To address this, different routes of administration for testosterone have been used and native testosterone replacement therapy has been licensed as implants, transdermal, transbuccal, and intranasal therapies (5).
Oral 17α-alkylated androgens such as methyltestosterone and oxymetholone were proved to be effective androgen replacement therapies but were associated with severe liver damage including the development of jaundice, peliosis hepatis, and liver tumors (6). This toxic effect on the liver appears to be specific to oral modified (i.e. non-native) testosterones, particularly methylated testosterone, and was not seen with native testosterone in animal models assessing liver toxicity (7). Testosterone undecanoate (TU) is an ester prodrug of testosterone and has a mid-chain length fatty acid at the 17β position and when given orally undergoes absorption in part through the intestinal lymphatic pathway, so circumventing some of the first-pass metabolism through the liver (4). Oral testosterone undecanoate is presented as an oily capsule and has been available in Europe since the 1970s (1); however, TU has to be taken with a meal two or three times daily, has an unpredictable absorption pattern, and generates high dihydrotestosterone (DHT) to testosterone ratio (8-10). An oral self-emulsifying formulation of TU has recently been approved in the US (Jatenzo®, Clarus Therapeutics Inc., USA). The formulation promotes solubilization and intestinal lymphatic absorption of the lipophilic testosterone ester. Deesterification of TU by nonspecific esterases in liver, blood, and tissue results in the production of testosterone. The liberated undecanoic acid moiety is metabolized via beta-oxidation. 5-Alpha reduction of testosterone undecanoate in the gut produces dihydrotestosterone undecanoate (DHTU) and DHT (11). The testosterone undecanoate formulation has to be taken with food, patients have higher than normal DHT levels on treatment and the label is associated with a black box warning regarding an increase in blood pressure (12). These data support the need for new developments in this area.
Various oral formulations of native testosterone have been tested in man although none have been licensed (13-20). Soon after testosterone’s identification and characterization, oral testosterone administration was disregarded as a viable route of administration and replacement because of poor oral absorption (21). In the 1970s, a micronized form of free testosterone was demonstrated to be absorbed in hypogonadal men but absorption was not reliable enough to progress as therapy (14). Further research, particularly by Amory and coworkers, showed that native testosterone administered as a suspension in oil, provided potentially therapeutic levels of testosterone in healthy men (15), and combined with 5αreductase inhibitors provided physiological testosterone levels both in the fasted and fed state (16). Native testosterone is practically insoluble in water and in fatty oil vehicles (22), and the challenge has been to develop a solution formulation that contains sufficient testosterone concentration to provide reproducible physiological testosterone levels in hypogonadal men. Building upon the previous observations, we have developed a lipidic solution formulation of native testosterone and have tested it in dogs and humans in the fasted and fed state.
Formulation: Lipidic Native Testosterone (NT) formulations were developed and assessed in vitro for dispersion behavior in gastric and intestinal media and for physical stability. A single formulation of NT, DITEST, was selected to take forward into pre-clinical trials (Table 1). The formulation used digestible lipids (oils with carbon chain length > 10 carbons atoms) with the addition of short-medium chain oils and ethanol as a polar co-solvent to assist with solubilization. The formulation was encapsulated in size 00 soft gelatin capsules with 40mg per capsule inside an aluminum foil blister pack and was stable for 2 years at ambient temperature (25°C).
Pharmacokinetics in hypogonadal men: Cohort 1, comparing 120mg NT with 80mg TU taken in the fed state with a high-fat meal showed both formulations generated testosterone levels in the physiological range and 80mg TU gave higher testosterone levels than 120mg NT (Figure 2). NT had an earlier Tmax than TU: 1.4 vs 4.2 hours (Table 4). NT resulted in around 50% lower levels of DHT than TU and the ratio of DHT: T for AUC0-10h for NT was 0.19 and for TU 0.36. Serum TU levels after dosing with 80mg TU were approximate ~10-fold greater than serum testosterone levels on a molar basis and showed considerable variability between subjects (Figure 3).
Cohort 2, NT 200mg given either fed with a high-fat meal or fasted showed similar testosterone levels and pharmacokinetics (figure 4). Comparing levels in cohort 2 to cohort 1, NT showed a serum testosterone AUC increase of 45% between 120mg and 200mg. NT 200mg fasted gave equivalent Cmax and AUC0-10h to TU 80mg fed: 90% CIs 88.0 (58.2 - 133.1) and 87.5 (54.6 - 140.2).
Cross-correlation of the PK parameters Cmax and AUCinf for serum testosterone levels after NT using all doses showed a weak negative correlation with bodyweight: r of -0.45 and -0.27 respectively.
There was one serious adverse event (urinary retention) during TU dosing. There were no emerging safety concerns and adverse event frequency and severity was similar between the different treatment arms.
Discussion
We have developed an oral lipidic formulation of native testosterone in a solution that provides physiological levels of testosterone and DHT when taken with or without food. The preclinical study in dogs showed that the oral lipidic NT formulation showed less variability in absorption between the fasted and fed state compared to TU and that very little TU was absorbed in the fasted state, confirming previous results in the literature (26). The results for the NT formulation was confirmed in hypogonadal men where the NT formulation showed similar pharmacokinetics when taken fasted or fed and the ratio of DHT to testosterone was lower for NT than TU.
It is known that native testosterone is absorbed orally but because of the extensive presystemic metabolism in the gastrointestinal tract and rapid first-pass metabolism in the liver, a high dose is required to replace physiological circulating serum testosterone levels (1). This is compounded by the fact testosterone is practically insoluble in water and fatty acid oils (22), so it has been challenging to generate a solution formulation of testosterone with a testosterone concentration sufficient to replace circulating testosterone levels. We have addressed this by generating a lipidic solution formulation where testosterone is held in solution in the oil phase through the addition of co-solvents: ethanol and benzyl alcohol. The formulation is stable at room temperature for up to 2 years and provides reproducible physiological testosterone levels in hypogonadal men.
*This manuscript reports clinical data from a single-dose study in a cohort of hypogonadal men and future studies will need to generate 24-hour pharmacokinetic data at steady state for a range of dose levels. Consideration will also need to be given to increasing the dose per capsule, measuring SHBG levels, and investigating the potential need for dose titration in clinical practice. Testosterone may induce its own metabolism and so the impact of repeat dosing will need to be examined (14). The levels of testosterone, DHT, and TU were quantified from serum samples, and following the start of the study in hypogonadal males it was recognized that TU can be converted to testosterone in serum ex vivo and therefore the testosterone levels measured after TU administration may be higher than they would have been if measured in plasma (28)
In conclusion, we have developed a lipidic NT formulation, which when given to hypogonadal men generates similar testosterone and DHT exposure in the fed and fasted state. Compared to published literature on a self-emulsifying formulation of TU at 200mg (12), the NT formulation at 200mg provides a similar testosterone Cmax and no requirement for a meal. This oral lipidic native testosterone formulation has anticipated advantages over current oral therapy of dosing with or without food and a lower risk of supraphysiological DHT levels.
Context: There is no licensed oral Native Testosterone (NT) because of challenges in the formulation. Licensed oral formulations of the ester, testosterone undecanoate (TU), require a meal for absorption and generate supraphysiological dihydrotestosterone (DHT) levels.
Objective: To develop an oral NT formulation.
Design and Methods: A lipid-based formulation of native testosterone-filled into soft-gelatin capsules at 40mg per capsule was designed with 2 years of stability at ambient temperature. Pharmacokinetic comparison studies of this oral lipidic NT formulation to oral TU were conducted in dogs and hypogonadal men.
Results: In dogs, 40mg NT was well absorbed under fasted conditions whereas 40mg TU required a high-fat meal: for NT the mean fed/fasted AUC ratio was 1.63 and for TU 7.05. In hypogonadal men fed and fasted NT had similar pharmacokinetics: Cmax mean 26.5 vs 30.4 nmol/l (769 vs 882 ng/dl), AUC0-10h 87 vs 88.6 h*nmol/l. NT (fed state) showed a testosterone AUC increase of 45% between 120mg and 200mg and NT 200mg gave a similar mean AUC0-10h to TU 80mg: 87 vs. 64.8 h*nmol/l. Serum TU levels were variable and on a molar basis were ~10-fold higher than serum testosterone levels after TU 80mg fed. The DHT: testosterone AUC0-10h ratio was more physiological for NT than TU being 0.19 vs 0.36. There were no emerging safety concerns with NT.
Conclusion: This novel oral lipidic native testosterone formulation has potential advantages over oral TU of dosing independently of food and a lower risk of supraphysiological DHT levels.
Significance Statement
There is no licensed oral testosterone because of challenges in formulation, and the oral formulations of the ester, testosterone undecanoate, require a fatty meal for absorption and generate supraphysiological dihydrotestosterone levels. We have overcome the design challenges and formulated oral native testosterone which can be taken with or without food and provides physiological levels of testosterone and dihydrotestosterone in hypogonadal men. This formulation, DITEST, has the potential advantage of being oral for patients who don’t tolerate injections and less risk of adverse events that might theoretically be associated with elevated dihydrotestosterone levels. Future studies will need to define the dosing regimen for replacement in hypogonadal men.
Introduction
Testosterone was isolated, named, and synthesized in 1935 (1), but to date, no oral native testosterone has been licensed for testosterone replacement therapy. The reason being that oral native testosterone, although absorbed through the intestine, undergoes extensive presystemic metabolism along the gastrointestinal tract (2), as well as rapid first-pass metabolism in the liver (3). The oral absorption of testosterone is also dependent on the dosing vehicle, wherein a lipophilic vehicle may increase the proportion of testosterone absorbed via the lymphatic route (4). It is thus difficult to achieve adequate bioavailability of testosterone in order to maintain consistent physiological testosterone levels via the oral route. To address this, different routes of administration for testosterone have been used and native testosterone replacement therapy has been licensed as implants, transdermal, transbuccal, and intranasal therapies (5).
Oral 17α-alkylated androgens such as methyltestosterone and oxymetholone were proved to be effective androgen replacement therapies but were associated with severe liver damage including the development of jaundice, peliosis hepatis, and liver tumors (6). This toxic effect on the liver appears to be specific to oral modified (i.e. non-native) testosterones, particularly methylated testosterone, and was not seen with native testosterone in animal models assessing liver toxicity (7). Testosterone undecanoate (TU) is an ester prodrug of testosterone and has a mid-chain length fatty acid at the 17β position and when given orally undergoes absorption in part through the intestinal lymphatic pathway, so circumventing some of the first-pass metabolism through the liver (4). Oral testosterone undecanoate is presented as an oily capsule and has been available in Europe since the 1970s (1); however, TU has to be taken with a meal two or three times daily, has an unpredictable absorption pattern, and generates high dihydrotestosterone (DHT) to testosterone ratio (8-10). An oral self-emulsifying formulation of TU has recently been approved in the US (Jatenzo®, Clarus Therapeutics Inc., USA). The formulation promotes solubilization and intestinal lymphatic absorption of the lipophilic testosterone ester. Deesterification of TU by nonspecific esterases in liver, blood, and tissue results in the production of testosterone. The liberated undecanoic acid moiety is metabolized via beta-oxidation. 5-Alpha reduction of testosterone undecanoate in the gut produces dihydrotestosterone undecanoate (DHTU) and DHT (11). The testosterone undecanoate formulation has to be taken with food, patients have higher than normal DHT levels on treatment and the label is associated with a black box warning regarding an increase in blood pressure (12). These data support the need for new developments in this area.
Various oral formulations of native testosterone have been tested in man although none have been licensed (13-20). Soon after testosterone’s identification and characterization, oral testosterone administration was disregarded as a viable route of administration and replacement because of poor oral absorption (21). In the 1970s, a micronized form of free testosterone was demonstrated to be absorbed in hypogonadal men but absorption was not reliable enough to progress as therapy (14). Further research, particularly by Amory and coworkers, showed that native testosterone administered as a suspension in oil, provided potentially therapeutic levels of testosterone in healthy men (15), and combined with 5αreductase inhibitors provided physiological testosterone levels both in the fasted and fed state (16). Native testosterone is practically insoluble in water and in fatty oil vehicles (22), and the challenge has been to develop a solution formulation that contains sufficient testosterone concentration to provide reproducible physiological testosterone levels in hypogonadal men. Building upon the previous observations, we have developed a lipidic solution formulation of native testosterone and have tested it in dogs and humans in the fasted and fed state.
Formulation: Lipidic Native Testosterone (NT) formulations were developed and assessed in vitro for dispersion behavior in gastric and intestinal media and for physical stability. A single formulation of NT, DITEST, was selected to take forward into pre-clinical trials (Table 1). The formulation used digestible lipids (oils with carbon chain length > 10 carbons atoms) with the addition of short-medium chain oils and ethanol as a polar co-solvent to assist with solubilization. The formulation was encapsulated in size 00 soft gelatin capsules with 40mg per capsule inside an aluminum foil blister pack and was stable for 2 years at ambient temperature (25°C).
Pharmacokinetics in hypogonadal men: Cohort 1, comparing 120mg NT with 80mg TU taken in the fed state with a high-fat meal showed both formulations generated testosterone levels in the physiological range and 80mg TU gave higher testosterone levels than 120mg NT (Figure 2). NT had an earlier Tmax than TU: 1.4 vs 4.2 hours (Table 4). NT resulted in around 50% lower levels of DHT than TU and the ratio of DHT: T for AUC0-10h for NT was 0.19 and for TU 0.36. Serum TU levels after dosing with 80mg TU were approximate ~10-fold greater than serum testosterone levels on a molar basis and showed considerable variability between subjects (Figure 3).
Cohort 2, NT 200mg given either fed with a high-fat meal or fasted showed similar testosterone levels and pharmacokinetics (figure 4). Comparing levels in cohort 2 to cohort 1, NT showed a serum testosterone AUC increase of 45% between 120mg and 200mg. NT 200mg fasted gave equivalent Cmax and AUC0-10h to TU 80mg fed: 90% CIs 88.0 (58.2 - 133.1) and 87.5 (54.6 - 140.2).
Cross-correlation of the PK parameters Cmax and AUCinf for serum testosterone levels after NT using all doses showed a weak negative correlation with bodyweight: r of -0.45 and -0.27 respectively.
There was one serious adverse event (urinary retention) during TU dosing. There were no emerging safety concerns and adverse event frequency and severity was similar between the different treatment arms.
Discussion
We have developed an oral lipidic formulation of native testosterone in a solution that provides physiological levels of testosterone and DHT when taken with or without food. The preclinical study in dogs showed that the oral lipidic NT formulation showed less variability in absorption between the fasted and fed state compared to TU and that very little TU was absorbed in the fasted state, confirming previous results in the literature (26). The results for the NT formulation was confirmed in hypogonadal men where the NT formulation showed similar pharmacokinetics when taken fasted or fed and the ratio of DHT to testosterone was lower for NT than TU.
It is known that native testosterone is absorbed orally but because of the extensive presystemic metabolism in the gastrointestinal tract and rapid first-pass metabolism in the liver, a high dose is required to replace physiological circulating serum testosterone levels (1). This is compounded by the fact testosterone is practically insoluble in water and fatty acid oils (22), so it has been challenging to generate a solution formulation of testosterone with a testosterone concentration sufficient to replace circulating testosterone levels. We have addressed this by generating a lipidic solution formulation where testosterone is held in solution in the oil phase through the addition of co-solvents: ethanol and benzyl alcohol. The formulation is stable at room temperature for up to 2 years and provides reproducible physiological testosterone levels in hypogonadal men.
*This manuscript reports clinical data from a single-dose study in a cohort of hypogonadal men and future studies will need to generate 24-hour pharmacokinetic data at steady state for a range of dose levels. Consideration will also need to be given to increasing the dose per capsule, measuring SHBG levels, and investigating the potential need for dose titration in clinical practice. Testosterone may induce its own metabolism and so the impact of repeat dosing will need to be examined (14). The levels of testosterone, DHT, and TU were quantified from serum samples, and following the start of the study in hypogonadal males it was recognized that TU can be converted to testosterone in serum ex vivo and therefore the testosterone levels measured after TU administration may be higher than they would have been if measured in plasma (28)
In conclusion, we have developed a lipidic NT formulation, which when given to hypogonadal men generates similar testosterone and DHT exposure in the fed and fasted state. Compared to published literature on a self-emulsifying formulation of TU at 200mg (12), the NT formulation at 200mg provides a similar testosterone Cmax and no requirement for a meal. This oral lipidic native testosterone formulation has anticipated advantages over current oral therapy of dosing with or without food and a lower risk of supraphysiological DHT levels.
