A “soft” analog is a congener of a longer acting drug that contains a metabolically labile chemical moiety that was specifically built into its molecular structure to allow rapid and predictable 1-step elimination.1 Such drugs commonly used in anesthesia practice include remifentanil and esmolol.2–4 Each of these soft drugs contains an ester moiety that is highly susceptible to nonspecific esterase hydrolysis and forms a carboxylic acid metabolite whose pharmacologic activity is orders of magnitude lower than that of the parent drug.
Several years ago, our laboratory began a research program to develop a soft analog of etomidate, a general anesthetic agent with relatively benign cardiovascular and respiratory side effects, but whose use is limited to single-bolus administration by its highly potent and prolonged suppression of adrenocortical steroid synthesis.5–12 Our primary goal was to create a drug that would retain etomidate’s highly desirable properties but whose effects on adrenocortical function would lift quickly after drug administration because the drug was rapidly eliminated. At the same time, it was hoped that recovery from the drug’s hypnotic action would similarly be very rapid even after prolonged infusion.
Methoxycarbonyl etomidate (MOC-etomidate) was the first soft etomidate analog (Fig. 1A).5 Similar to remifentanil and esmolol, it contains a metabolically labile ester moiety that is linked to the pharmacophore via a 2-carbon spacer. This spacer facilitates hydrolysis by esterases by reducing steric hindrance around the ester. Preclinical studies in rats demonstrated that after single-bolus administration or very brief (e.g., 5 minutes) continuous infusions, MOC-etomidate had an ultrashort duration of hypnotic action (~1 minute) and did not produce persistent adrenocortical suppression.5,6 However, with longer infusions (e.g., 30 minutes) that required the administration of larger quantities of MOC-etomidate, hypnotic recovery slowed dramatically as MOC-etomidate’s carboxylic acid metabolite (MOC-ECA) accumulated to levels in both the blood and cerebrospinal fluid (CSF) that were sufficient to produce hypnosis.10
In an attempt to produce soft etomidate analogs devoid of such context sensitivity, we developed a series of second-generation soft etomidate analogs in which the chemical structure of the spacer varied.8 Of these new compounds, cyclopropyl methoxycarbonyl metomidate (CPMM) possessed the most promising pharmacology in animals (Fig. 1B) because it exhibited the highest potency and produced hypnosis that reversed within several minutes after stopping continuous infusions lasting as long as 2 hours.7,8,11 CPMM is now undergoing trials in humans.
The purpose of this study was to assess the pharmacology of CPMM’s carboxylic acid metabolite (CPMM-CA). We synthesized CPMM-CA and then defined its potency for directly activating γ-aminobutyric acid type A (GABAA) receptors and producing hypnosis in tadpoles. We also measured the extent to which CPMM-CA accumulated in the blood and CSF with a 2-hour continuous CPMM infusion. We then compared these results with those obtained with MOC-etomidate and its metabolite. Our goal was to better understand why hypnotic recovery after CPMM infusion is context insensitive, whereas hypnotic recovery after MOC-etomidate infusion is not context insensitive. Our hypothesis was that hypnotic recovery after CPMM (as opposed to MOC-etomidate) infusion is context insensitive because its metabolite fails to reach concentrations in either the blood or the CSF that are sufficient to have a hypnotic effect.
All animal studies were conducted with the approval of the Subcommittee on Research Animal Care at the Massachusetts General Hospital, Boston, Massachusetts. Xenopus laevis tadpoles (early prelimb stage) and adult female X laevis frogs were purchased from Xenopus One (Ann Arbor, MI) and housed in our laboratory (tadpoles) or in the Massachusetts General Hospital Center for Comparative Medicine animal care facility (frogs). Adult male Sprague-Dawley rats (330–430 g) were purchased from Charles River Laboratories (Wilmington, MA) and caged in the animal care facility of the Massachusetts General Hospital Center for Comparative Medicine. CPMM was administered through a 24-gauge venous catheter placed in a tail vein. Blood and CSF samples were taken from a femoral arterial catheter and an intracisternal cannula, respectively, implanted by the vendor before delivery.
MOC-etomidate, MOC-ECA, and CPMM were synthesized by Aberjona Laboratories (Woburn, MA) using our previously published methods.5,6,8 CPMM-CA was synthesized using the 2-step scheme shown in Figure 2.
To a solution of 1 (2.6 g, 12 mol) in dichloromethane (100 mL) was added (COCl)2 (1.6 mL) and dimethylformamide (4 drops) at 0°C dropwise. The reaction mixture was stirred at room temperature until completion of the reaction as monitored by high-performance liquid chromatography (HPLC). The reaction mixture was concentrated and azeotroped 3 times with anhydrous toluene. The crude product 2 was then dried on a vacuum pump for 3 hours for use in step 2 without storage.
To a solution of 3 (1.22 g, 12 mmol) in dichloromethane (200 mL) was added triethylamine (6 mL) and 4-dimethylaminopyridine (catalyst), followed by 2 from step 1 (12 mmol) in dichloromethane (40 mL) at 0°C. The reaction mixture was stirred at 0°C for 2 hours and then at room temperature overnight. The reaction mixture was concentrated in vacuo. Tetrahydrofuran was added to the residue and acidified with diluted aqueous HCl to pH = 3. The mixture was concentrated, and the residue was purified by preparative HPLC to yield the product 4 (1.1 g; >98% purity).
Mass spectroscopy (M + 1 = 301), 1H NMR (400 MHz, dimethyl sulfoxide-d6) δ 9.23 (s, 1H), 8.27 (s, 1H), 7.27 to 7.40 (m, 5H), 6.27 (q, J = 7.2 Hz, 1H), 1.90 (d, J = 7.2 Hz, 3H), 1.41 to 1.44 (m, 2H), 1.11 to 1.31 (m, 2H).
GABAA Receptor Electrophysiology
Oocytes were harvested from frogs as previously described and injected with mRNA encoding the α1(L264T), β3, and γ2 subunits of the human GABAA receptor (5 ng of mRNA total at a subunit ratio of 1:1:3).5 As previously reported, we incorporate the L264T ion channel mutation into the receptor because it stabilizes the receptor’s open state, thus increasing anesthetic sensitivity and allowing more complete anesthetic concentration-response curves to be generated before reaching aqueous saturation at high anesthetic concentrations and without the potentially confounding effects of a coadministered agonist.13–15 Oocytes were then incubated in ND96 buffer (96 mM NaCl, 2 mM KCl, 1.8 mM CaCl2, 1 mM MgCl2, 5 mM HEPES, pH = 7.4) containing 0.05 mg/mL gentamicin for at least 18 hours at 18°C before study.
Electrophysiologic recordings were performed using the whole-cell 2-electrode voltage-clamp technique as previously described.14 Peak current amplitudes elicited by a 30-second application of the desired compound were normalized to control currents elicited by 100 μM GABA in the same oocyte. For each compound, the concentration-mean peak current response data were fit to a Hill equation with minima and maxima constrained to 0% and 100%, respectively, to obtain the median effective concentration (EC50) for direct activation of GABAA receptors. From these data, a potency ratio was defined as the EC50 of the metabolite relative to the EC50 for the parent hypnotic. For each compound, the concentration range studied was limited by its solubility in the buffer. Data reported for CPMM are from previously published studies by our laboratory using the same approach.12
Tadpole Loss-of-Righting Reflexes Assay
Loss-of-righting reflexes (LoRR) in X laevis tadpoles was assessed as previously described.5,16 In brief, groups of 5 tadpoles were placed in water buffered with 2.5 mM Tris HCl (pH = 7.4) containing the desired concentration of CPMM-CA. Tadpoles were tipped every 5 minutes with a flame-polished pipette until the response stabilized (20–30 minutes). A tadpole was determined to have LoRR if it failed to right itself within 5 seconds after being turned supine. At the end of each study, tadpoles were returned to fresh water to ensure reversibility. The concentration-mean response data for LoRR were fit to a Hill equation with minima and maxima constrained to 0% and 100%, respectively, to obtain the EC50 for LoRR. From these data, a potency ratio was defined as the EC50 of the parent hypnotic relative to the EC50 for the metabolite. The concentration range of CPMM-CA studied was limited by its solubility in the buffer. Data reported for MOC-etomidate, MOC-ECA, and CPMM are from previously published studies by our laboratory using the same approach.5,12,15
CPMM Infusion Protocol
CPMM was solubilized in saline (8 mg/mL) and infused IV for 2 hours using our previously published protocol.7 This protocol induces and maintains an electroencephalographic burst suppression ratio of 80% in the presence of 1% isoflurane and produces a steady-state anesthetic depth that is 1.3 times that required to produce immobility to a standard noxious stimulus in the absence of isoflurane. The total dose of CPMM delivered during these 2-hour infusions was 71 mg/kg. Comparative data reported for MOC-etomidate are from previously published 30-minute infusion studies by our laboratory using a protocol that also induces and maintains an electroencephalographic burst suppression ratio of 80% in the presence of 1% isoflurane and produces a steady-state anesthetic depth that is 1.3 times that required to produce immobility to a standard noxious stimulus in the absence of isoflurane.10
Measurement of Blood and CSF CPMM-CA Concentrations
Before, during, and after CPMM infusion, blood samples (200 μL/sample) were intermittently collected through the femoral arterial catheter and immediately mixed with acetonitrile (200 μL). This blood was replaced with an equal volume of normal saline. The samples were then centrifuged, and the resultant plasma was collected and stored at −20°C until analyzed. After thawing, the CPMM-CA concentration in each sample was determined by HPLC using an Agilent Technologies 1200 system with a Poroshell 120, 4.6 × 50 mm, 2.7 μm C18 column (Santa Clara, CA) with the ultraviolet detector set to 242 nm. A linear gradient 20% to 90% acetonitrile in water >30 minutes was used with a flow rate of 1 mL/min. Standards were made by dissolving the desired quantity of CPMM-CA in methanol.
Before, during, and after CPMM infusion, CSF samples (25 μL/sample) were collected through the intracisternal catheter, immediately mixed with acetonitrile (50 μL), and stored at −20°C until analyzed. The concentration of CPMM-CA in CSF was assessed by HPLC as described earlier for the blood.
A standard curve was constructed using 8 CPMM-CA concentrations ranging from 1 to 100 μM. The relationship between the peak area and the CPMM-CA concentration was linear with an r2 > 0.99. The HPLC assay’s lower limit of quantitation for CPMM-CA along with its precision and accuracy was determined using guidelines established by the International Conference on Harmonisation.17 Intraday and interday precision and accuracy were assessed at 5, 10, and 20 μM CPMM-CA (n = 27 HPLC measurements over 3 days). At these 3 concentrations, the relative SDs (for both intraday and interday precision) were <5%, 3%, and 2%, respectively. The accuracy of all samples (both intraday and interday) were within 13% of their nominal values. The lower limit of quantitation was calculated from the SD of the response and the slope of the calibration curve to be 0.7 μM.
Measurement of MOC-Etomidate and CPMM In Vitro Metabolic Half-Life in Brain Tissue
The metabolic half-lives of MOC-etomidate and CPMM were quantified using a previously described protocol.10 Pooled rat brain homogenate (20 mg/mL in phosphate-buffered saline media, pH 7.4) was purchased from Bioreclamation LLC (Hicksville, NY). MOC-etomidate or CPMM (120 μL from a 1 mM stock solution in saline) was added to 1.08 mL of the homogenate. After the desired incubation period at 37°C, a 100-μL aliquot was removed, and the metabolism was stopped by mixing with 100 μL of acetonitrile. The samples were then centrifuged, and the supernatant was collected and stored at −20°C until analyzed by HPLC as described above for in vivo studies. The metabolic half-lives of MOC-etomidate and CPMM in rat brain homogenate were calculated from the incubation time-dependent reduction in MOC-etomidate and CPMM concentrations assuming a first-order process and complete metabolism at infinite time.
All data are reported as mean ± SD unless otherwise noted. In each study, the number of repetitions used to define the mean value of each data point (6 for oocyte studies, 5 for tadpole studies, and 3 for all other studies) was defined before initiating the study and was based on the precision observed from previous studies from our laboratory. Curve fitting for concentration-response curves (GABAA receptor and tadpole studies) was performed with Prism 6.0 (Graphpad, La Jolla, CA), and potency was quantified as an EC50 and a 95% confidence interval (CI). The 95% CI for each EC50 was calculated using error propagation. Metabolic half-lives in rat brain homogenate were quantified (along with a 99% CI) and statistically compared using an extra sum-of-squares F test with Prism 6.0.
Modulation of GABAA Receptor Function
Both hypnotics (MOC-etomidate and CPMM) and their carboxylic acid metabolites (MOC-ECA and CPMM-CA, respectively) directly activated GABAA receptors in a concentration-dependent manner (Fig. 3). Figure 3A plots the relationship between the MOC-etomidate and MOC-ECA concentrations and the normalized peak current amplitude response. A fit of the data to a Hill equation yielded an EC50 for direct activation of 20 μM (95% CI, 16–25 μM) for MOC-etomidate and 8300 μM (95% CI, 6280–11,100 μM) for MOC-ECA, yielding a potency ratio between metabolite and parent hypnotic of 1:415 (95% CI, 1:260–1:665). Figure 3B plots the same relationship for CPMM and CPMM-CA. A fit of the data to a Hill equation yielded an EC50 for direct activation of 3.8 μM (95% CI, 2.8–5.0 μM) for CPMM and 18,800 μM (95% CI, 9460–37,300 μM) for CPMM-CA, yielding a potency ratio between metabolite and parent hypnotic of 1:4900 (95% CI, 1:1540–1:16,340).
LoRR in Tadpoles
Both hypnotics and their carboxylic acid metabolites induced LoRR in tadpoles in a concentration-dependent manner (Fig. 4) and without lethality. Figure 4A plots the relationship between the MOC-etomidate and MOC-ECA concentrations and the fraction of tadpoles that had LoRR. A fit of the data to a Hill equation yielded an EC50 for LoRR of 7.5 μM (95% CI, 5.2–10.7 μM) for MOC-etomidate and 2900 μM (95% CI, 1660–5020 μM) for MOC-ECA, yielding a potency ratio between metabolite and parent hypnotic of 1:390 (95% CI, 1:103–1:1445). Figure 4B plots the same relationship for CPMM and CPMM-CA. A fit of the data to a Hill equation yielded an EC50 for LoRR of 2.6 μM (95% CI, 2.1–3.1 μM) for CPMM and 5000 μM (95% CI, 4700–5300 μM) for CPMM-CA, yielding a potency ratio between metabolite and parent hypnotic of 1:1900 (95% CI, 1:1330–1:2890).
Metabolite Concentrations in Blood on Hypnotic Infusion
CPMM was continuously infused for 2 hours. The CPMM-CA concentration was 26 ± 10 μM measured in the first blood sample drawn 30 minutes after initiating that infusion and remained near that value (mean, 31 ± 6.8 μM; range, 26–39 μM) throughout the 2-hour infusion period (Fig. 5). Sixty minutes after the infusion ended, the CPMM-CA concentration in the blood decreased to 1.5 ± 0.3 μM. For comparison, Figure 5 also shows the blood concentrations of MOC-ECA measured in our previous study using a 30-minute infusion of MOC-etomidate that achieved a similar hypnotic depth as judged by both behavioral and electroencephalographic metrics.10 By the end of the 30-minute infusion, the metabolite concentration reached 1170 ± 48 μM, a value that is approximately 45-fold higher than that reached on infusing CPMM for the same duration of time and achieving the same anesthetic depth. After the MOC-etomidate infusion ended, the MOC-ECA concentration in the blood decreased to 820 ± 42 μM over the next 60 minutes and then to 480 ± 110 μM when measured 4 hours after the MOC-etomidate infusion stopped.
Metabolite Concentrations in CSF on Hypnotic Infusion
In our previous MOC-etomidate infusion studies in rats, we found that MOC-ECA poorly penetrated the blood–brain barrier but nevertheless reached a CSF concentration that was twice that present in the blood (2500 ± 110 μM).10 We concluded that most of this MOC-ECA was formed from MOC-etomidate that had diffused across the blood–brain barrier and was then metabolized in situ by esterases present in central nervous system tissues (e.g., brain). Once formed within the central nervous system, such MOC-ECA was trapped there for a prolonged period of time because it was charged at physiologic pH. It was within this context that we measured CPMM-CA concentrations in the CSF during and after 2-hour CPMM infusions. On the basis of those studies with MOC-etomidate, we had expected that the CPMM-CA concentration in the CSF would similarly be approximately twice that present in the blood (i.e., ~50 μM). However, we found no quantifiable CPMM-CA in the CSF at any time point during the study (data not shown). This implies that even with infusions of CPMM lasting 2 hours, the CPMM-CA concentration in the CSF was below our 0.7 μM limit of quantitation.
Metabolic Half-Lives in Brain Tissue: MOC-Etomidate Versus CPMM
The unexpectedly low levels of CPMM-CA in the CSF led us to hypothesize that CPMM is metabolized more slowly than MOC-etomidate by esterases within the central nervous system, and, therefore, less of its metabolite accumulates there. To test this hypothesis, we compared the rates with which the 2 hypnotics were metabolized by rat brain tissue homogenate. Figure 6 shows that CPMM was metabolized significantly more slowly than MOC-etomidate with half-lives of 1414 (99% CI, 997–2,427 minutes) and 96 minutes (99% CI, 92–101 minutes), respectively.
The primary goal of these studies was to characterize the pharmacology of CPMM-CA and to compare it with that of MOC-ECA so that we could better understand why hypnotic recovery after infusions of CPMM are context insensitive, whereas those after MOC-etomidate are not. Our studies showed that similar to MOC-ECA, CPMM-CA enhances GABAA receptor function and produces LoRR in tadpoles in a concentration-dependent manner. However, CPMM-CA’s potency in these 2 assays relative to its parent hypnotic was approximately 1:4900 and 1:1900, respectively, whereas that for MOC-ECA was only approximately 1:415 and 1:390, respectively. Our studies also revealed that, with 2-hour CPMM infusions, CPMM-CA reached respective concentrations in the blood and CSF that were 2 and >3 orders of magnitude lower than those which produce hypnosis (in tadpoles). These results may be contrasted with our previous studies showing that with just 30-minute MOC-etomidate infusions, MOC-ECA reached concentrations in the blood and CSF that approximated those that produce hypnosis.10 Finally, we show that CPMM is metabolized significantly more slowly than MOC-etomidate by esterases found in the brain, which may explain why the concentration of CPMM-CA in the CSF is so much lower than that in the blood.
A key concept in the design of soft analogs is that the potency of the metabolite must be significantly lower than that of the parent drug to allow the pharmacologic action to rapidly terminate once the parent drug has been metabolized.1,18,19 For soft drugs that are continually infused for a prolonged period of time, this potency ratio must be particularly large because their metabolites typically accumulate in the body during the infusion period and therefore could produce persistent pharmacologic effects. In the cases of remifentanil and esmolol, the metabolite:parent drug potency ratios have been estimated to be as high as 1:4600 for the former and between 1:1600 and 1:1900 for the latter.20,21 Studies suggest that these potency ratios are sufficiently large to avoid a pharmacologically significant impact of the metabolite (GR90291 for remifentanil and ASL-8123 for esmolol) even when its elimination is slowed by renal failure.22,23
The potency ratio for CPMM:CPMM-CA is approximately an order of magnitude greater than that of MOC-etomidate:MOC-ECA in our assays and approximates those of remifentanil:GR90291 and esmolol:ASL-8123. Thus, it is possible to infuse CPMM (but not MOC-etomidate) at hypnotic doses for a prolonged length of time without metabolite concentrations reaching levels in either the blood or the CSF that produce hypnosis. By analogy to remifentanil and esmolol, we expect that this would be true even in the presence of renal failure because the CPMM:CPMM-CA potency ratio is so large.
Although CPMM and MOC-etomidate are metabolized in rat blood at similar rates (half-lives: 0.57 and 0.41 minutes, respectively), these studies show that they are metabolized in rat brain tissue at rates that differ by 15-fold (half-lives: 1414 and 96 minutes, respectively).8 Such substrate selectivity is not uncommon among esterases and may be leveraged to fine tune the pharmacology of soft drugs.24–26 This finding also suggests that the esterases responsible for metabolizing these drugs in the brain are different from those in the blood. We believe that CPMM’s relatively low rate of metabolism in the brain (and presumably low blood–brain barrier permeability because it is a carboxylic acid) explains why CPMM-CA’s concentration in the CSF was so much lower than in blood: CPMM-CA is poorly produced in situ by central nervous system tissue (e.g., brain) and little of the CPMM-CA that is produced in the periphery (e.g., by the blood and the liver) crosses the blood–brain barrier to reach the central nervous system. This suggests that even had the potency ratio of CPMM:CPMM-CA been closer to that of MOC-etomidate:MOC-ECA, CPMM-CA concentrations in the central nervous system might still have been insufficient to have a hypnotic effect.
Although we did not characterize the elimination of CPMM-CA or MOC-ECA in any detail, we note that 60 minutes after stopping infusion of the parent hypnotic, the blood concentration of CPMM-CA decreased by 94%, whereas that of MOC-ECA decreased by only 30%. Four hours after stopping the MOC-etomidate infusion, the blood concentration of MOC-ECA was still comparatively high having decreased by only 60%. The renal excretion of anionic compounds (including carboxylic acids) from the blood is saturable at high blood concentrations because it is dependent on active tubular secretion.27 Therefore, the relatively slow elimination of MOC-ECA (compared with CPMM-CA) may be most simply explained by its nearly 2 orders of magnitude higher blood concentration.
Hypnotic recovery after administration of CPMM, as opposed to MOC-etomidate, is context insensitive because its carboxylic acid metabolite does not accumulate to hypnotic levels in the central nervous system even after prolonged infusion. This low level of accumulation reflects (1) the >1000-fold potency ratio between CPMM and its metabolite and (2) the relative resistance of CPMM to metabolism by esterases in the brain.
Name: Ervin Pejo, BS.
Contribution: This author performed the experiments.
Attestation: Ervin Pejo approved the final manuscript and attests to the integrity of the original data and the analysis reported in this manuscript.
Conflicts of Interest: The author declares no conflicts of interest.
Name: Jifeng Liu, PhD.
Contribution: This author oversaw the synthesis and purification of CPMM-CA.
Attestation: Jifeng Liu approved the final manuscript.
Conflicts of Interest: The author declares no conflicts of interest.
Name: Xiangjie Lin, MSc
Contribution: This author synthesized and purified CPMM-CA.
Attestation: Xiangjie Lin approved the final manuscript.
Conflicts of Interest: The author declares no conflicts of interest.
Name: Douglas E. Raines, MD.
Contribution: This author designed the experiments, analyzed the data, and wrote the manuscript.
Attestation: Douglas E. Raines approved the final manuscript and attests to the integrity of the original data and the analysis reported in this manuscript, and is the archival author.
Conflicts of Interest: Douglas E. Raines is an inventor on patent applications submitted and held by the Massachusetts General Hospital. He, his department, his laboratory, and his institution could receive royalties relating to the development of cyclopropyl-methoxycarbonyl metomidate or related analogs. He is a consultant for The Medicines Company, which has licensed technologies covered by those patents.
This manuscript was handled by: Ken B. Johnson, MD.
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