Paliperidone Palmitate Long-Acting Injectable Given Intramuscularly in the Deltoid Versus the Gluteal Muscle

I n the report by Yin et al, 1 the statement that “many clinicians may misinterpret these directions (in paliperidone palmitate product monograph) to mean that these intramuscular sites (deltoid or gluteal) are interchangeable, and thus therapeutically equivalent” is, in our opinion, not correct. We are writing to clarify some statements from the Yin et al article concerning the deltoid versus gluteal intramuscular injection of paliperidone palmitate. The recommended initiation as stated in the product label of paliperidone palmitate 1-month injection is with a dose of 150mgeqon treatmentday1and100mgeq1 week later, both administered in the deltoid muscle. Here, the deltoid and gluteal injection sites are not interchangeable, and from a clinical point of view, it is important to reach therapeutic concentrations quickly when starting a new antipsychotic. However, after the second initiation dose, monthly maintenance doses can be administered in either the deltoid or gluteal muscle. The injection sites of the maintenance doses can be used interchangeably, but for deltoid injections, the appropriate needle length is based on the patient's weight (1-in 23G needle for patients weighing <90 kg and 1.5-in 22G needle for patients weighing ≥90 kg). For gluteal injection, regardless of patient weight, the use of a 1.5-in 22G needle is recommended. The use of differently sized needles depending on the weight of the patient for injection into the deltoid muscle is recommended to limit the risk of drug administration into the subcutaneous tissues, which would result in a decreased rate of absorption from the intramuscular site of injection. Different distributions of muscle and adipose tissue, as well as difference in blood flow between the deltoid and gluteal sites, may affect the uptake rate of paliperidone into the circulation from the site of injection. At the deltoid site, the likelihood of an injection that is purely intramuscular is higher compared with that at the gluteal injection site. The hypovascularity of subcutaneous adipose tissue compared with