Clinical data | |||
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Trade names | Adipex-P, Ionamin, Suprenza, others | ||
Other names | α,α-Dimethylphenethylamine; α,α-Dimethylphenylethylamine; α-Methylamphetamine | ||
AHFS/Drugs.com | Monograph | ||
MedlinePlus | a682187 | ||
Pregnancy category |
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Dependence liability | Physical: Not typical Psychological: Moderate [1] | ||
Addiction liability | Low [2] | ||
Routes of administration | By mouth [3] | ||
Drug class | Psychostimulant; Appetite suppressant; [3] Norepinephrine–dopamine releasing agent | ||
ATC code | |||
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Legal status |
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Pharmacokinetic data | |||
Bioavailability | High (almost 100%) [5] | ||
Protein binding | 17.5% [6] | ||
Metabolism | Minimal (6%) [6] | ||
Elimination half-life | 20–25 hours, urinary pH-dependent [6] [5] | ||
Excretion | Urine (62–85% unchanged) [5] [6] | ||
Identifiers | |||
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CAS Number | |||
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DrugBank | |||
ChemSpider | |||
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KEGG | |||
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CompTox Dashboard (EPA) | |||
ECHA InfoCard | 100.004.112 | ||
Chemical and physical data | |||
Formula | C10H15N | ||
Molar mass | 149.237 g·mol−1 | ||
3D model (JSmol) | |||
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Phentermine, sold under the brand name Adipex-P among others, is a medication used together with diet and exercise to treat obesity. [3] It is available by itself or as the combination phentermine/topiramate. [7] Phentermine is taken by mouth. [3]
Common side effects include a fast heart beat, high blood pressure, trouble sleeping, dizziness, and restlessness. [3] Serious side effects may include abuse, but do not include pulmonary hypertension or valvular heart disease, as the latter complications were caused by the fenfluramine component of the "fen-phen" combination. [3] Phentermine is an norepinephrine and dopamine releasing agent (NDRA) and produces stimulant, rewarding, and appetite suppressant effects. [8] [9] [10] Chemically, it is a substituted amphetamine. [11]
Phentermine was approved for medical use in the United States in 1959. [3] It is available as a generic medication. [3] In 2022, it was the 149th most commonly prescribed medication in the United States, with more than 3 million prescriptions. [12] [13] Phentermine was withdrawn from the market in the United Kingdom in 2000, while the combination medication fen-phen, of which it was a part, was withdrawn from the market in 1997 due to side effects of fenfluramine. [14]
Phentermine is used for a short period of time to promote weight loss, if exercise and calorie reduction are not sufficient, and in addition to exercise and calorie reduction. [5] [15]
Phentermine is approved for up to 12 weeks of use and most weight loss occurs in the first weeks. [15] However, significant loss continues through the sixth month and has been shown to continue at a slower rate through the ninth month. [16]
Use is not recommended during pregnancy or breastfeeding, [17] or with selective serotonin reuptake inhibitors (SSRIs) or monoamine oxidase inhibitors (MAOIs). [3]
Phentermine is contraindicated for users who: [5] [15]
Tolerance usually occurs; however, risks of dependence and addiction are considered negligible. [16] [18] People taking phentermine may be impaired when driving or operating machinery. [15] Consumption of alcohol with phentermine may produce adverse effects. [15]
There is currently no evidence regarding whether or not phentermine is safe for pregnant women. [5] [15]
Other adverse effects include: [5] [15]
Phentermine may decrease the effect of drugs like clonidine, methyldopa, and guanethidine. Drugs to treat hypothyroidism may increase the effect of phentermine. [15]
Compound | NE | DA | 5-HT | Ref | ||
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Phenethylamine | 10.9 | 39.5 | >10,000 | [19] [20] [21] | ||
Dextroamphetamine | 6.6–7.2 | 5.8–24.8 | 698–1,765 | [22] [23] | ||
Dextromethamphetamine | 12.3–13.8 | 8.5–24.5 | 736–1,292 | [22] [24] | ||
Phentermine | 28.8–39.4 | 262 | 2,575–3,511 | [22] [21] [25] | ||
Chlorphentermine | >10,000 (RI) | 935–2,650 | 18.2–30.9 | [22] [25] | ||
Notes: The smaller the value, the more strongly the drug releases the neurotransmitter. The assays were done in rat brain synaptosomes and human potencies may be different. See also Monoamine releasing agent § Activity profiles for a larger table with more compounds. Refs: [8] [9] |
Phentermine is a substrate of the monoamine transporters (MATs) and acts as a monoamine releasing agent (MRA), specifically as a norepinephrine–dopamine releasing agent (NDRA). [8] [9] [10] It also acts as a norepinephrine–dopamine reuptake inhibitor (NDRI) to a lesser extent. [8] [10] The drug robustly and dose-dependently elevates brain norepinephrine and dopamine levels in animals. [8] Phentermine is more potent in its effects on norepinephrine than on dopamine and the drug shows only weak effects on serotonin. [8] [9] [10] Unlike many other amphetamines and MRAs, phentermine is completely inactive at the vesicular monoamine transporter 2 (VMAT2). [19] [26] Due to its actions on the catecholamines, phentermine produces effects including stimulation, rewarding effects, appetite suppression, and sympathomimetic effects in animals and humans. [8] [6]
In terms of monoamine release in-vitro using rat brain synaptosomes, phentermine is about 6-fold less potent than dextroamphetamine in the case of norepinephrine release, 11-fold less potent than dextroamphetamine in the case of dopamine release, and has a ratio of norepinephrine release versus dopamine release of about 6.6:1 compared to dextroamphetamine's ratio of about 3.5:1. [8] [9] [10] It is more than 3-fold less potent than amphetamine in elevating brain dopamine and serotonin levels in rodents in vivo , is about 10-fold less potent than amphetamine in terms of self-administration in monkeys, and is a relatively weak reinforcer in rodents. [10] [27] Although phentermine induces the release of dopamine at sufficiently high concentrations in vitro and at sufficiently high doses in rodents and monkeys in vivo, it may result in only weak or negligible brain dopamine release in humans at typical clinical doses. [9] This may be due to its selectivity for induction of norepinephrine over dopamine release and may be analogous to the case of ephedrine (which is at least 10-fold selective for induction of norepinephrine over dopamine release). [9] The effects of phentermine may be more related to noradrenergic activation rather than dopaminergic activity. [9] However, more research is needed to assess the preceding notions. [9]
As with other MRAs, phentermine produces dopaminergic neurotoxicity in rodents at high doses. [10] It can also produce serotonergic neurotoxicity at very high doses in rodents. [10] The clinical significance of these findings for humans, in which employed doses may be much lower, are unclear. [10]
The combination of phentermine with a serotonin releasing agent (SRA) like fenfluramine results in suppression of brain dopamine release by phentermine and marked attenuation or abolition of phentermine's stimulant and rewarding effects in animals and humans. [8] [10] [27] [28] [29] [30] [31] Conversely, combined phentermine and fenfluramine administration synergistically enhances the appetite suppression of these drugs in animals and results in greater weight loss than either drug alone in humans. [10] Fenfluramine produces serotonergic neurotoxicity in animals and addition of phentermine results in either no change or augmentation of this neurotoxicity. [10]
Phentermine has been found to be completely inactive as a ligand or agonist of the serotonin 5-HT2 receptors, including of the serotonin 5-HT2A, 5-HT2B, and 5-HT2C receptors. [9] [10] [27] This is in contrast to the serotonin releasing agents (SRAs) fenfluramine, norfenfluramine, and to a lesser extent chlorphentermine. [9] [27] However, another study found that phentermine was a weak human serotonin 5-HT2C receptor partial agonist (EC50 = 1,394 nM, Vmax = 66%). [27] In accordance with its lack of serotonin release and serotonin 5-HT2B receptor agonism, phentermine appears to show no risk of primary pulmonary hypertension (PPH) or valvular heart disease (VHD) in humans. [32] [10] [27]
Phentermine has been found to be active as an agonist of the rat and human trace amine-associated receptor 1 (TAAR1). [33] [34] [35] [36] It appears to be a weak human TAAR1 partial agonist (EC50 = 5,470 nM and Emax = 68% in one study). [34] The drug shows reduced activity as a TAAR1 agonist compared to amphetamine. [34] [36] TAAR1 agonism by amphetamines that possess this action may serve to auto-inhibit and constrain their effects. [37] [38] [39] [40]
Phentermine is a very weak monoamine oxidase inhibitor (MAOI) in vitro . [41] [10] [42] It specifically inhibits monoamine oxidase A (MAO-A) (IC50 = 85,000–143,000 nM) and monoamine oxidase B (MAO-B) (IC50 = 285,000 nM). [41] [10] [42] However, its potency as a MAOI is far below its potency as a monoamine releasing agent. [10] Relatedly, phentermine does not show neurochemical signs of MAOI activity in rodents in vivo . [10] As such, the significance of phentermine as an MAOI in humans is questionable. [10]
The pharmacokinetics of phentermine are dose-dependent. [6] [6] Peak concentrations of phentermine are reached 6 hours following oral administration of a dose of 15 mg. [6] The steady-state levels of phentermine with continuous administration have been found to be around 200 ng/mL in clinical studies. [6] The oral bioavailability of phentermine is not affected by intake of a high-fat meal. [6]
The volume of distribution of phentermine is 5 L/kg. [6] Its plasma protein binding is approximately 17.5%. [6]
Phentermine undergoes minimal metabolism. [6] Only about 6% of an administered dose of phentermine is metabolized. [6] It is metabolized to a minor extent by para-hydroxylation, N-oxidation, and N-hydroxylation, followed by conjugation. [6]
The drug is eliminated mainly in urine. [6] It is excreted 62 to 85% unchanged in urine. [6] [6] The elimination half-life of phentermine is 20 to 25 hours. [6] [5] The elimination of phentermine is modified by urine acidicity or pH. [6] [5] In the case of acidic urine (pH < 5), the elimination half-life of phentermine has been found to be 7 to 8 hours. [6] The clearance of phentermine is 8.79 L/h. [6]
In 1959, phentermine first received approval from the US Food and Drug Administration (FDA) as an appetite suppressant. [43] Eventually a hydrochloride salt and a resin form became available. [43]
Phentermine was marketed with fenfluramine or dexfenfluramine as a combination appetite suppressant and fat burning agent under the popular name fen-phen. [44] In 1997, after 24 cases of heart valve disease in fen-phen users, fenfluramine and dexfenfluramine were voluntarily taken off the market at the request of the FDA. [45] Studies later showed nearly 30% of people taking fenfluramine or dexfenfluramine for up to 24 months had abnormal valve findings. [46]
Phentermine is still available by itself in most countries, including the US. [43] However, because it is similar to amphetamine, it is classified as a controlled substance in many countries. Internationally, phentermine is a schedule IV drug under the Convention on Psychotropic Substances. [47] In the United States, it is classified as a Schedule IV controlled substance under the Controlled Substances Act. In contrast, amphetamine preparations are classified as Schedule II controlled substances. [48]
A company called Vivus developed a combination drug, phentermine/topiramate that it originally called Qnexa and then called Qsymia, which was invented and used off-label by Thomas Najarian, who opened a weight-clinic in Los Osos, California in 2001; Najarian had previously worked at Interneuron Pharmaceuticals, which had developed one of the fen-phen drugs previously withdrawn from the market. [49] The FDA rejected the combination drug in 2010 due to concerns over its safety. [49] In 2012 the FDA approved it after Vivus re-applied with further safety data. [50] At the time, one obesity specialist estimated that around 70% of his colleagues were already prescribing the combination off-label. [49]
Phentermine, also known as α,α-dimethylphenethylamine or as α-methylamphetamine, is a substituted phenethylamine and amphetamine. [51] [52] [53] It is the derivative of amphetamine in which a second methyl group is present at the alpha carbon. [11] [51] [52] [53] The drug is a positional isomer of methamphetamine (N-methylamphetamine) and of other methylamphetamines such as 4-methylamphetamine. [51] [52] [53]
A number of derivatives of phentermine exist, including cericlamine, chlorphentermine, cloforex, clortermine, etolorex, mephentermine, 3,4-methylenedioxyphentermine (MDPH), 3,4-methylenedioxy-N-methylphentermine (MDMP or MDMPH), and pentorex, among others. [52] [53] Some of these drugs, including chlorphentermine, cloforex, clortermine, and mephentermine, have been marketed as pharmaceutical drugs similarly to phentermine, for instance as appetite suppressants. [52] [53]
The term ‘phentermine' is contracted from phenyl-tertiary-butyl amine.
It is marketed under many brand names and formulations worldwide, including Acxion, Adipex, Adipex-P, Duromine, Elvenir, Fastin, Ionamin, Lomaira (phentermine hydrochloride), Panbesy, Qsymia (phentermine and topiramate), Razin, Redusa, Sentis, Suprenza, and Terfamex. [54]
The drug combination fenfluramine/phentermine, usually called fen-phen, is an anti-obesity medication that is no longer widely available. It was sold in the early 1990s, and utilized two anorectics. Fenfluramine was marketed by American Home Products as Pondimin, but was shown to cause potentially fatal pulmonary hypertension and heart valve problems, which eventually led to its withdrawal in 1997 and legal damages of over $13 billion. Phentermine was not shown to have harmful effects.
3,4-Methylenedioxyamphetamine (MDA), sometimes referred to as sass, is an empathogen-entactogen, stimulant, and psychedelic drug of the amphetamine family that is encountered mainly as a recreational drug. In its pharmacology, MDA is a serotonin–norepinephrine–dopamine releasing agent (SNDRA). In most countries, the drug is a controlled substance and its possession and sale are illegal.
Fenfluramine, sold under the brand name Fintepla, is a serotonergic medication used for the treatment of seizures associated with Dravet syndrome and Lennox–Gastaut syndrome. It was formerly used as an appetite suppressant in the treatment of obesity, but was discontinued for this use due to cardiovascular toxicity before being repurposed for new indications. Fenfluramine was used for weight loss both alone under the brand name Pondimin and in combination with phentermine commonly known as fen-phen.
Phenmetrazine, sold under the brand name Preludin among others, is a stimulant drug first synthesized in 1952 and originally used as an appetite suppressant, but withdrawn from the market in the 1980s due to widespread misuse. It was initially replaced by its analogue phendimetrazine which functions as a prodrug to phenmetrazine, but now it is rarely prescribed, due to concerns of misuse and addiction. Chemically, phenmetrazine is a substituted amphetamine containing a morpholine ring or a substituted phenylmorpholine.
4-Methylaminorex is a stimulant drug of the 2-amino-5-aryloxazoline group that was first synthesized in 1960 by McNeil Laboratories. It is also known by its street name "U4Euh" ("Euphoria"). It is banned in many countries as a stimulant. 4-Methylaminorex has effects comparable to methamphetamine but with a longer duration.
Aminorex, sold under the brand names Menocil and Apiquel among others, is a weight loss (anorectic) stimulant drug. It was withdrawn from the market after it was found to cause pulmonary hypertension (PPH). In the United States, aminorex is a Schedule I controlled substance.
Chlorphentermine, sold under the brand names Apsedon, Desopimon, and Lucofen, is a serotonergic appetite suppressant of the amphetamine family. Developed in 1962, it is the para-chloro derivative of the better-known appetite suppressant phentermine, which is still in current use.
Etilamfetamine, also known as N-ethylamphetamine and formerly sold under the brand names Apetinil and Adiparthrol, is a stimulant drug of the amphetamine family. It was invented in the early 20th century and was subsequently used as an anorectic or appetite suppressant in the 1950s, but was not as commonly used as other amphetamines such as amphetamine, methamphetamine, and benzphetamine, and was largely discontinued once newer drugs such as phenmetrazine were introduced.
Naphthylaminopropane, also known as naphthylisopropylamine (NIPA), is an experimental drug that was under investigation for the treatment of alcohol and stimulant addiction.
Norfenfluramine, or 3-trifluoromethylamphetamine, is a never-marketed drug of the amphetamine family and a major active metabolite of the appetite suppressants fenfluramine and benfluorex. The compound is a racemic mixture of two enantiomers with differing activities, dexnorfenfluramine and levonorfenfluramine.
para-Chloroamphetamine (PCA), also known as 4-chloroamphetamine (4-CA), is a serotonin–norepinephrine–dopamine releasing agent (SNDRA) and serotonergic neurotoxin of the amphetamine family. It is used in scientific research in the study of the serotonin system, as a serotonin releasing agent (SRA) at lower doses to produce serotonergic effects, and as a serotonergic neurotoxin at higher doses to produce long-lasting depletions of serotonin.
A monoamine releasing agent (MRA), or simply monoamine releaser, is a drug that induces the release of one or more monoamine neurotransmitters from the presynaptic neuron into the synapse, leading to an increase in the extracellular concentrations of the neurotransmitters and hence enhanced signaling by those neurotransmitters. The monoamine neurotransmitters include serotonin, norepinephrine, and dopamine; MRAs can induce the release of one or more of these neurotransmitters.
A serotonin releasing agent (SRA) is a type of drug that induces the release of serotonin into the neuronal synaptic cleft. A selective serotonin releasing agent (SSRA) is an SRA with less significant or no efficacy in producing neurotransmitter efflux at other types of monoamine neurons, including dopamine and norepinephrine neurons.
A norepinephrine releasing agent (NRA), also known as an adrenergic releasing agent, is a catecholaminergic type of drug that induces the release of norepinephrine (noradrenaline) and epinephrine (adrenaline) from the pre-synaptic neuron into the synapse. This in turn leads to increased extracellular concentrations of norepinephrine and epinephrine therefore an increase in adrenergic neurotransmission.
A serotonin–dopamine releasing agent (SDRA) is a type of drug which induces the release of serotonin and dopamine in the body and/or brain.
4-Methylamphetamine (4-MA), also known by the former proposed brand name Aptrol, is a stimulant and anorectic drug of the amphetamine family. It is structurally related to mephedrone (4-methylmethcathinone).
4-Methylmethamphetamine (4-MMA), also known as mephedrine, is a putative stimulant and entactogen drug of the amphetamine family. It acts as a serotonin–norepinephrine–dopamine releasing agent (SNDRA). The drug is the β-deketo analogue of mephedrone and the N-methyl analogue of 4-methylamphetamine (4-MA).
Substituted cathinones, or simply cathinones, which include some stimulants and entactogens, are derivatives of cathinone. They feature a phenethylamine core with an alkyl group attached to the alpha carbon, and a ketone group attached to the beta carbon, along with additional substitutions. Cathinone occurs naturally in the plant khat whose leaves are chewed as a recreational drug.
Levofenfluramine (INN), or (−)-3-trifluoromethyl-N-ethylamphetamine, also known as (−)-fenfluramine or (R)-fenfluramine, is a drug of the amphetamine family that, itself (i.e., in enantiopure form), was never marketed. It is the levorotatory enantiomer of fenfluramine, the racemic form of the compound, whereas the dextrorotatory enantiomer is dexfenfluramine. Both fenfluramine and dexfenfluramine are anorectic agents that have been used clinically in the treatment of obesity (and hence, levofenfluramine has been as well since it is a component of fenfluramine). However, they have since been discontinued due to reports of causing cardiovascular conditions such as valvular heart disease and pulmonary hypertension, adverse effects that are likely to be caused by excessive stimulation of 5-HT2B receptors expressed on heart valves.
4,4'-Dimethylaminorex, sometimes referred to by the street name "Serotoni", is a psychostimulant and entactogen designer drug related to aminorex, 4-methylaminorex, and pemoline. It was first detected in the Netherlands in December 2012, and has been sold as a designer drug around Europe since mid-2013.
RESULTS. Methamphetamine and amphetamine potently released NE (IC50s = 14.3 and 7.0 nM) and DA (IC50s = 40.4 nM and 24.8 nM), and were much less potent releasers of 5-HT (IC50s = 740 nM and 1765 nM). Phentermine released all three biogenic amines with an order of potency NE (IC50 = 28.8 nM)> DA (IC50 = 262 nM)> 5-HT (IC50 = 2575 nM). [...] Chlorphentermine was a very potent 5-HT releaser (IC50 = 18.2 nM), a weaker DA releaser (IC50 = 935 nM) and inactive in the NE release assay. Chlorphentermine was a moderate potency inhibitor of [3H]NE uptake (Ki = 451 nM). [...]
we confirmed agonistic activity at human TAAR1 of several other compounds, including the trace amines octopamine and tryptamine, the amphetamine derivatives l-amphetamine, d-methamphetamine, (+)-MDMA, and phentermine, and the catecholamine metabolites 3-MT and 4-MT (Bunzow et al., 2001; Lindemann and Hoener, 2005; Reese et al., 2007; Wainscott et al., 2007; Wolinsky et al., 2007; Xie and Miller, 2007; Xie et al., 2007).
Interestingly, the concentrations of amphetamine found to be necessary to activate TAAR1 are in line with what was found in drug abusers [3, 51, 52]. Thus, it is likely that some of the effects produced by amphetamines could be mediated by TAAR1. Indeed, in a study in mice, MDMA effects were found to be mediated in part by TAAR1, in a sense that MDMA auto-inhibits its neurochemical and functional actions [46]. Based on this and other studies (see other section), it has been suggested that TAAR1 could play a role in reward mechanisms and that amphetamine activity on TAAR1 counteracts their known behavioral and neurochemical effects mediated via dopamine neurotransmission.
Another feature that distinguishes [synthetic cathinones (SCs)] from amphetamines is their negligible interaction with the trace amine associated receptor 1 (TAAR1). Activation of this receptor reduces the activity of dopaminergic neurones, thereby reducing psychostimulatory effects and addictive potential (Miller, 2011; Simmler et al., 2016). Amphetamines are potent agonists of this receptor, making them likely to self‐inhibit their stimulating effects. In contrast, SCs show negligible activity towards TAAR1 (Kolaczynska et al., 2021; Rickli et al., 2015; Simmler et al., 2014, 2016). [...] It is worth noting, however, that for TAAR1 there is considerable species variability in its interaction with ligands, and it is possible that the in vitro activity of [rodent TAAR1 agonists] may not translate into activity in the human body (Simmler et al., 2016). The lack of self‐regulation by TAAR1 may partly explain the higher addictive potential of SCs compared to amphetamines (Miller, 2011; Simmler et al., 2013).
β-Keto-analogue cathinones also exhibited approximately 10-fold lower affinity for the TA1 receptor compared with their respective non-β-keto amphetamines. [...] Activation of TA1 receptors negatively modulates dopaminergic neurotransmission. Importantly, methamphetamine decreased DAT surface expression via a TA1 receptor-mediated mechanism and thereby reduced the presence of its own pharmacological target (Xie and Miller, 2009). MDMA and amphetamine have been shown to produce enhanced DA and 5-HT release and locomotor activity in TA1 receptor knockout mice compared with wild-type mice (Lindemann et al., 2008; Di Cara et al., 2011). Because methamphetamine and MDMA auto-inhibit their neurochemical and functional effects via TA1 receptors, low affinity for these receptors may result in stronger effects on monoamine systems by cathinones compared with the classic amphetamines.