Entecavir hydrate
[CAS 209216-23-9]

Identification

• Classification: API >> Synthetic anti-infective drugs >> Antiviral drugs
• Name: Entecavir hydrate
• Synonyms: 2-Amino-1,9-dihydro-9-[(1S,3R,4S)-4-hydroxy-3-(hydroxymethyl)-2-methylenecyclopentyl]-6H-purin-6-one monohydrate
• Molecular Formula: C₁₂H₁₅N₅O₃.H₂O
• Molecular Weight: 295.30
• CAS Registry Number: 209216-23-9
• EC Number: 606-668-5
• SMILES: C=C1[C@H](C[C@@H]([C@H]1CO)O)N2C=NC3=C2N=C(NC3=O)N.O

Properties

• Density: 1.81 g/mL (Expl.)
• Solubility: DMSO: <80mg/mL Water: <1.2mg/mL, Ethanol: <1.2mg/mL (Expl.)

Safety Data

• Hazard Symbols: GHS07;GHS08 Danger
• Risk Statements: H302-H336-H351-H360-H361-H361d-H362-H372
• Safety Statements: P203-P260-P261-P263-P264-P270-P271-P280-P301+P317-P304+P340-P318-P319-P330-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Acute toxicity	Acute Tox.	4	H302
Specific target organ toxicity - repeated exposure	STOT RE	1	H372
Carcinogenicity	Carc.	2	H351
Reproductive toxicity	Repr.	2	H361
Reproductive toxicity	Repr.	1B	H360
Reproductive toxicity	Lact.	-	H362

chemBlink Chemical Story

Entecavir hydrate, CAS 209216-23-9, is the monohydrate form of entecavir, a synthetic guanosine analogue used against hepatitis B virus. PubChem identifies the hydrate as C12H17N5O4 and explicitly describes it as entecavir monohydrate. The parent antiviral belongs to the nucleoside reverse-transcriptase inhibitor class and is selectively active against hepatitis B virus.

Inside infected cells, entecavir is phosphorylated to its active triphosphate. That metabolite competes with the natural substrate deoxyguanosine triphosphate for hepatitis B virus polymerase. The result is inhibition of several polymerase functions required for viral DNA synthesis. This biochemical strategy resembles the broader nucleoside-analogue concept: build a molecule similar enough to a natural nucleoside to be processed by cellular enzymes, but different enough to interfere with viral replication after activation.

Entecavir emerged from medicinal-chemistry work at Bristol-Myers Squibb and was known during development as BMS-200475. Its carbocyclic framework replaces the oxygen normally found in the sugar ring of a nucleoside with carbon, helping create a metabolically robust analogue while preserving the spatial arrangement needed for phosphorylation and enzyme recognition. It later became an important oral therapy for chronic hepatitis B.

The word "hydrate" describes a different level of chemistry. One molecule of water is incorporated in the defined crystalline material. Hydration can influence solid-state properties relevant to pharmaceutical processing and quality control even though the active molecular framework remains entecavir. PubChem consequently links the hydrate to entecavir as the parent compound and lists water as the other component.

This distinction between molecular pharmacology and crystal form is important. The antiviral mechanism belongs to entecavir triphosphate inside the cell; the monohydrate designation belongs to the solid drug substance before administration. Both identities matter, but for different reasons.

References:
1. PubChem, Entecavir Hydrate, CID 135526609, CAS 209216-23-9.
2. FDA, Baraclude (entecavir) prescribing information.
3. Yamanaka G et al. Antimicrob Agents Chemother. 1999;43:190-193.
4. Innaimo SF et al. Antimicrob Agents Chemother. 1997;41:1444-1448.

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