Which long‑acting injectable antipsychotics are administered as a single intramuscular injection once every four weeks for maintenance treatment in adults?

Long-Acting Injectable Antipsychotics Administered Once Every 4 Weeks

The second-generation long-acting injectable antipsychotics administered as a single intramuscular injection once every four weeks include risperidone LAI, paliperidone palmitate, olanzapine pamoate, and aripiprazole once-monthly (AOM 400). 1, 2, 3, 4, 5

Available Monthly LAI Formulations

Risperidone Long-Acting Injectable

• Risperidone LAI is administered every 2 weeks or can be dosed every 4 weeks depending on the formulation, and represents the first depot formulation of an atypical antipsychotic prepared by encapsulating risperidone into biodegradable microspheres. 3
• This agent is recommended by the American Psychiatric Association as a first consideration when initiating long-acting injectable treatment, particularly for patients with recurrent relapses related to non-adherence or as first-line maintenance treatment after the first episode of schizophrenia. 2

Paliperidone Palmitate

• Paliperidone palmitate is formulated as an aqueous suspension of nanocrystal molecules and can be administered once monthly after initial loading doses. 3
• This formulation has demonstrated efficacy in protecting patients from psychotic, depressive, and manic symptoms in both schizophrenia and schizoaffective disorder. 6

Olanzapine Pamoate (OLAI)

• Olanzapine pamoate is a microcrystalline salt of olanzapine and pamoic acid suspended in aqueous solution, administered every 2 or 4 weeks without oral supplementation. 3, 5
• This represents the most direct transition option for patients already stabilized on oral olanzapine, as it maintains the same pharmacological profile the patient has already demonstrated tolerability to. 1
• A critical caveat: OLAI requires a risk management plan with observation by qualified personnel in a healthcare facility for at least 3 hours post-injection due to the possibility of post-injection syndrome (occurring in <0.1% of injections). 5

Aripiprazole Once-Monthly (AOM 400)

• Aripiprazole 400 mg once-monthly LAI is a lyophilized powder with an elimination half-life of 29.9-46.5 days, allowing for a 4-week injection interval. 4
• This is the first dopamine D2 partial agonist available in a long-acting formulation, with documented efficacy in a 12-week double-blind trial and two maintenance-of-effect trials (38-week and 52-week). 4
• The 400 mg dose is tolerated by >90% of patients, with adverse events typically mild or moderate (increased weight, akathisia, injection site pain, sedation occurring ≥5% and at least twice that for placebo). 4
• A key advantage: injection does not require additional training of health personnel or post-injection observation, unlike olanzapine pamoate. 4

Clinical Selection Algorithm

Step 1: Assess Prior Oral Response

• The primary criterion for choosing an LAI is the patient's prior response and tolerability to a specific oral antipsychotic; select the corresponding LAI formulation when available. 1
• For patients already stabilized on oral olanzapine, olanzapine pamoate represents the most direct transition. 1
• For patients on risperidone or requiring a D2 antagonist profile, risperidone LAI or paliperidone palmitate are appropriate choices. 1

Step 2: Consider Practical Factors

• When multiple LAIs are appropriate, incorporate the patient's expressed preference and the pharmacokinetic characteristics of each formulation (dosing interval, steady-state time). 1
• For patients who cannot comply with 3-hour post-injection observation, avoid olanzapine pamoate and select aripiprazole once-monthly, risperidone LAI, or paliperidone palmitate instead. 4, 5

Step 3: Timing of Initiation

• Initiate LAI therapy as early as possible after acute psychotic symptoms have improved and dosage flexibility is no longer needed. 1
• During the acute phase, use oral antipsychotics or short-acting intramuscular injections to allow dose adjustments; introduce LAIs only after clinical stabilization. 1

Evidence Quality and Guideline Support

• Second-generation LAIs are preferred over first-generation agents due to better overall tolerability and fewer neurological side effects (extrapyramidal symptoms). 7, 2, 3
• Current evidence does not demonstrate superiority of any single LAI regarding efficacy, although side-effect profiles differ among agents. 1
• The principal clinical advantage of all LAIs is improved adherence, which translates into reduced relapse rates and fewer psychiatric hospitalizations through guaranteed medication delivery. 7