⚠️They are the most used class of medication in the USA and the 07th in the UK.
The first one discovered in 1937 was a compound called 933F, followed shortly after by 929F.
⛔The big problem is that both were very toxic to the human being.
❗In 1942 Antergan and in 1945 Diphynhidramine were discovered.
‼️Second generation were only developed in the 1980s.
✅Currently, there are over 40 different ones.
🔷️Often people divide them between drowsy and non-drowsy.
🔷️Some others call them slow acting or fast acting.
🔷️The correct name is first generation and second generation.
🔹️Saying this, there is some debate about a potential third generation.
🔹️To make matters slightly more confusing, some second generation also have first generation properties.
✅But we are only talking about the ones acting on H1 receptors.
✳ These are used for all sorts of allergic reactions:
▶️Insect venom allergy
▶️Allergy to chemicals
▶️Urticaria or angioedema
❓What are the main differences?
➡️All 1st generation and a couple of second generation need to be metabolized in the liver, to become active.
➡️1st generation have less afinity for the H1 receptors, than other generations. ➡️2nd generation reaches peak plasma level and onset of action much faster than 1st generation.
➡️2nd generation are mainly excreted unchanged in urine (Cetirizine) or faeces (Fexofenadine).
➡️1st generation are less specific for H1 receptors and pass the blood-brain barrier, leading to sedating effects.
➡️Some 2nd generation can also be used topically on the nose or eyes (Azelastine), having a faster effect than when taken by mouth, but must be applied several times a day.
✴In reality, there are 4 different types of histamine receptors (from H1 to H4).
1️⃣H1 was described in 1966
2️⃣H2 was described in 1972
3️⃣H3 was described in 1983
4️⃣H4 was described in 1994
❇We have antagonists (actual name is reverse agonists) for H1 and H2, but not for the others.
🔻H1 are further divided into 7 different chemical groups.
Dr Costa is a Consultant Paediatrician and fellow of the Royal College of Paediatrics and Child Health.