Levomequitazine is the L-enantiomer of mequitazine. The antihistaminergic activity mainly resides in the S-enantiomer, L-mequitazine, whereas the anticholinergic activity mainly resides in the D-enantiomer. It was shown, that L-enantiomer of mequitazine is less potent antagonist of human M3 muscarinic acetylcholine receptors than D-enantiomer. In vitro binding studies have shown that the affinity of L-mequitazine for H1 receptors is approximately ten times higher and to muscarinic receptors ten times lower, compared to d-mequitazine. Memory impairment was observed after administration of L-mequitazine 10 mg alone on delayed recall. This could be due to indirect effects of H1 receptor blockade. L-mequitazine 10 mg produced mild driving impairment, whereas L-mequitazine 2.5 and 5.0 mg show no effects on driving. Levomequitazine had been in phase III clinical trials by Pierre Fabre for the treatment of perennial allergic rhinitis and seasonal allergic rhinitis.

Mapinastine (E 4716) is a selective antagonist of histamine H1 receptors. In animal models, mapinastine exerts antiallergic and anti-asthmatic properties. It demonstrated a lack of central effects. Mapinastine development has been discontinued.

Cipralisant (GT-2331) is an H3-receptor ligand developed by Gliatech in the late 1990s. The drug exhibits complex pharmacology: it behaves as an antagonist in a guinea pig jejunum contraction assay and mouse disogenia model, as a partial agonist in a rat brain synaptosome model, and as a full agonist at recombinant H3 receptors. In preclinical models, cipralisant enhanced wakefulness and improved learning in developmental rat models. In 2000 cipralisant was investigated in the clinical trials for the treatment of attention-deficit hyperactivity disorder, but due to the bankruptcy of Gliatech the development of the drug was discontinued.

Vapitadine (R129160; Hivenyl™) is an antihistamine that Barrier Therapeutics is developing as a treatment for allergic reactions of the skin, such as those associated with hives and for the itch associated with atopic dermatitis. Vapitadine is a new selective, non-sedative H1antihistamine. In several in vitro and in vivo pharmacological models, vapitadine is at least as potent as cetirizine (Zyrtec®) and does not penetrate the blood-brain barrier. An advantage of vapitadine over other antihistamines may be the absence of the sedation, even at high doses. Vapitadine had been in phase II clinical trials for the treatment of patients with atopic dermatitis. However, no development has been reported.

IRDABISANT is a histamine H3 receptor inverse agonist with potential therapeutic utility in cognition enhancement.

Tiotidine is a controversial histamine H2 receptor ligand with negligible activity against H1- and H3- receptors. It was found that tiotidine behaves as an inverse agonist in U-937 cells, diminishing basal cAMP levels. Tiotidine showed two binding sites, one with high affinity and low capacity and the other with low affinity and high capacity. Tiotidine is currently in use as a radioligand in histamine H2-receptor binding studies. Compared to cimetidine, tiotidine appears to be approximately eight times more potent on a molar basis than cimetidine as an inhibitor of acid secretion, and the tiotidine effect is more prolonged. It was developed for the treatment of peptic ulcer.

Tecastemizole (aka Norastemizole) is an H1 receptor antagonist that showed great promise as potential next-generation antihistamine allergy medication. Sepracor developed Norastemizole through phase III clinical trials; however, the FDA rejected the application for approval on the grounds of an unacceptable profile of adverse side-effects including phospholipidosis and cardiomyopathies in animals exposed to the drug.