Histamine H1 receptors are antagonistic to mepyraminemaleate. For H1, H2, and H3 receptors, the Kd values are 0.8 nM, 5200 nM, and >3000 nM, in that order [1]. The H1 receptor has a pKd of 9.4 [2]. In the brains of guinea pigs (0.8 nM), rats (9.1 nM), DDT1-MF-2 and BC3H1 cells (276 nM), and rat brains, mepyramine binds to H1 receptors with distinct Kd [1]. Mepyramine lowers the maximal response to ATP in CHO-gpH1 cells and lowers InsP levels in CHO-gpH1 cells with a log EC50 of -7.94 ± 0.11 [3].

Intraperitoneal injections of mepyramine at doses of 10 and 20 mg/kg considerably suppressed the second-stage nociceptive response in rats, but injections at a value of 5 mg/kg had no discernible effect [4].

Absorption, Distribution and Excretion
The H1 antagonists are well absorbed from the gi tract. Following oral administration, peak plasma concn are achieved in 2 to 3 hr and effects usually last 4 to 6 hr; however, some of the drugs are much longer acting ... . /Histamine Antagonists: H1 Antagonists/
... H1 antagonists are eliminated more rapidly by children than by adults and more slowly in those with severe liver disease. /Histamine Antagonists: H1 Antagonists/

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Metabolism / Metabolites
MAIN SITE OF METABOLIC TRANSFORMATION IS LIVER. /ANTIHISTAMINES/
H1 blockers are among the many drugs that induce hepatic microsomal enzymes, and they may facilitate their own metabolism. /Histamine Antagonists: H1 Antagonists/

Effects During Pregnancy and Lactation
◉ Summary of Use during Lactation
Small, occasional doses of pyrilamine are probably acceptable during breast feeding. Larger doses or more prolonged use may cause effects in the infant or decrease the milk supply, particularly, in combination with a sympathomimetic such as pseudoephedrine or before lactation is well established. The nonsedating antihistamines are preferred alternatives.
◉ Effects in Breastfed Infants
Relevant published information on pyrilamine was not found as of the revision date. In one telephone follow-up study, mothers reported irritability and colicky symptoms 10% of infants exposed to various antihistamines and drowsiness was reported in 1.6% of infants. None of the reactions required medical attention and none of the infants were exposed to pyrilamine.
◉ Effects on Lactation and Breastmilk
Antihistamines in relatively high doses given by injection can decrease basal serum prolactin in nonlactating women and in early postpartum women. However, suckling-induced prolactin secretion is not affected by antihistamine pretreatment of postpartum mothers. Whether lower oral doses of antihistamines have the same effect on serum prolactin or whether the effects on prolactin have any consequences on breastfeeding success have not been studied. The prolactin level in a mother with established lactation may not affect her ability to breastfeed.

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Interactions
Concurrent use /of ototoxic medications/ with antihistamines may mask the symptoms of ototoxicity such as tinnitus, dizziness, or vertigo. /Antihistamines/
Concurrent use of monoamine oxidase (MAO) inhibitors with antihistamines may prolong and intensify the anticholinergic and CNS depressant effects of antihistamines; concurrent use is not recommended. /Antihistamines/
Concurrent use /with alcohol or other CNS depression-producing medications/ may potentiate the CNS depressant effects of either these medications or antihistamines; also, concurrent use of maprotiline or tricyclic antidepressants may potentiate the anticholinergic effects of either antihistamines or these medications. /Antihistamines/
Anticholinergic effects may be potentiated when /anticholinergics or other medications with anticholinergic activity/ are used concurrently with antihistamines; patients should be advised to report occurrence of gastrointestinal problems promptly since paralytic ileus may occur with concurrent therapy. /Antihistamines/
Concurrent use /of other photosensitizing medications/ with antihistamines may cause additive photosensitizing effects. /Antihistamines/

[1]. Hill SJ. Distribution, properties, and functional characteristics of three classes of histamine receptor. Pharmacol Rev. 1990 Mar;42(1):45-83.

[2]. Selective ligands as tools to study histamine receptors. Eur J Med Chem. 2000 Jan;35(1):5-20.

[3]. Mepyramine, a histamine H1 receptor inverse agonist, binds preferentially to a G protein-coupled form of the receptor and sequesters G protein. J Biol Chem. 2004 Aug 13;279(33):34431-9.

[4]. Effects of mepyramine and famotidine on the physostigmine-induced antinociception in the formalin test in rats. Pak J Biol Sci. 2008 Nov 15;11(22):2573-8.

Pyrilamine is a viscous brown liquid. (NTP, 1992)
Mepyramine is an ethylenediamine derivative that is ethylenediamine in which one of the amino nitrogens is substituted by two methyl groups and the remaining amino nitrogen is substituted by a 4-methoxybenzyl and a pyridin-2-yl group. It has a role as a H1-receptor antagonist. It is an ethylenediamine derivative and an aromatic ether.
Mepyramine, or pyrilamine, targets the H1 receptor. It is a first generation antihistamine. However, it rapidly permeates the brain and so often causes drowsiness as a side effect. It has been found in over-the-counter combination products for colds and menstrual symptoms, but is considered to be an unapproved prescription medication used for cough, cold, or allergic conditions.
A histamine H1 antagonist. It has mild hypnotic properties and some local anesthetic action and is used for allergies (including skin eruptions) both parenterally and locally. It is a common ingredient of cold remedies.
See also: Pyrilamine Maleate (has salt form).

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Drug Indication
Indicated for the treatment of allergic conditions, symptomatic relief of hypersensitivity reaction, and treatment of pruritic skin disorders.

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Mechanism of Action
Mepyramine is a histamine H1 receptor inverse agonist. It binds to a G protein-coupled form of the receptor and promotes a G protein-coupled inactive state of the H1 receptor that interferes with the Gq/11-mediated signaling. Mepyramine competes with histamine for binding at H₁-receptor sites on the effector cell surface, resulting in suppression of histaminic edema, flare, and pruritus. The sedative properties of Mepyramine occur at the subcortical level of the CNS.
Antihistamines used in the treatment of allergy act by competing with histamine for H1-receptor sites on effector cells. They thereby prevent, but do not reverse, responses mediated by histamine alone. Antihistamines antagonize, in varying degrees, most of the pharmacological effects of histamine, including urticaria and pruritus. Also, the anticholinergic actions of most antihistamines provide a drying effect on the nasal mucosa. /Antihistamines/
H1 antagonists inhibit most responses of smooth muscle to histamine. Antagonism of the constrictor action of histamine on respiratory smooth muscle is easily shown in vivo and in vitro. /Histamine Antagonists: H1 Antagonists/
H1 antagonists strongly block the action of histamine that results in increased permeability and formation of edema and wheal. /Histamine Antagonists: H1 Antagonists/
Some H1 antagonists possess local anesthetic activity ... . /Histamine Antagonists: H1 Antagonists/
For more Mechanism of Action (Complete) data for PYRILAMINE (6 total), please visit the HSDB record page.

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Therapeutic Uses
Anti-Allergic Agents; Histamine H1 Antagonists
Antihistamines are indicated in the prophylactic and symptomatic treatment of perennial and seasonal allergic rhinitis, vasomotor rhinitis, and allergic conjunctivitis due to inhalant allergens and foods. /Antihistamines; Included in US product labeling/
Antihistamines are indicated for the symptomatic treatment of pruritus associated with allergic reactions and of mild, uncomplicated allergic skin manifestations of urticaria and angioedema, in dermatographism, and in urticaria associated with transfusions. /Antihistamines; Included in US product labeling/
Antihistamines are also used in the treatment of pruritus associated with pityriasis rosea. /Antihistamines; NOT included in US product labeling/
For more Therapeutic Uses (Complete) data for PYRILAMINE (11 total), please visit the HSDB record page.

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Drug Warnings
Use is not recommended in newborn or premature infants because this age group has an increased susceptibility to anticholinergic side effects, such as central nervous system excitation, and an increased tendency toward convulsions. A paradoxical reaction characterized by hyperexcitability may occur in children taking antihistamines. /Antihistamines/
Dizziness, sedation, confusion, and hypotension may be more likely to occur in geriatric patients taking antihistamines. Geriatric patients are especially susceptible to the anticholinergic side effects, such as dryness of mouth and urinary retention (especially in males), of the antihistamines. If these side effects occur and continue or are severe, medication should probably be discontinued. /Antihistamines/
Prolonged use of antihistamines ... may decrease or inhibit salivary flow, thus contributing to the development of caries, periodontal disease, oral candidiasis, and discomfort. /Antihistamines/
H1 antagonists are most useful in acute exudative types of allergy that present with symptoms of rhinitis, urticaria, and conjunctivitis. Their effect, however, is purely palliative and confined to the suppression of symptoms attributable to the histamine-antibody reaction. The drugs do not diminish the intensity of this reaction, which is the cause of the various hypersensitivity diseases. /Histamine Antagonist: H1 Antagonists/
For more Drug Warnings (Complete) data for PYRILAMINE (11 total), please visit the HSDB record page.

C21H27N3O5

401.4562

401.195

59-33-6

59-33-6;91-84-9;6036-95-9 (HCl);

4992

White to off-white solid powder

423.8ºC at 760 mmHg

100-101ºC

210.1ºC

2.37

0

4

7

21

277

0

YECBIJXISLIIDS-UHFFFAOYSA-N

InChI=1S/C17H23N3O/c1-19(2)12-13-20(17-6-4-5-11-18-17)14-15-7-9-16(21-3)10-8-15/h4-11H,12-14H2,1-3H3

N'-[(4-methoxyphenyl)methyl]-N,N-dimethyl-N'-pyridin-2-ylethane-1,2-diamine

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: Please store this product in a sealed and protected environment (e.g. under nitrogen), avoid exposure to moisture and light.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : ≥ 100 mg/mL (~249.09 mM)
H2O : ~50 mg/mL (~124.55 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (6.23 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (6.23 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (6.23 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: 100 mg/mL (249.09 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication.

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 (Please use freshly prepared in vivo formulations for optimal results.)