Bevacizumab, known as Avastin, is a medication administered as an intravitreal injection directly into the vitreous humor, the clear gel that fills the eye. This targeted delivery ensures the medication reaches the back of the eye where disease activity occurs. The duration of its biological presence and its clinical effect are two distinct measures that determine the patient’s retreatment schedule.
Avastin’s Mechanism in Ocular Conditions
The injection combats serious eye conditions characterized by the growth of abnormal blood vessels and fluid leakage. Avastin functions as an anti-VEGF agent, specifically targeting Vascular Endothelial Growth Factor (VEGF). VEGF is a protein that, when overproduced in the eye, signals the growth of new, fragile, and leaky blood vessels.
These abnormal vessels occur in conditions such as wet Age-related Macular Degeneration (AMD), Diabetic Macular Edema (DME), and Retinal Vein Occlusion (RVO). When injected, Avastin binds to and neutralizes the VEGF protein, blocking the signal that drives vessel growth and fluid accumulation. Halting this process stabilizes vision and prevents further damage to the retina and macula.
Ocular Half-Life and Duration of Action
To determine how long Avastin “stays” in the eye, clinicians look at its half-life, which is the time required for the concentration of the drug in the vitreous humor to decrease by half. Studies suggest the estimated biological half-life of Avastin following an intravitreal injection is typically in the range of four to seven days. This relatively rapid clearance means the amount of active drug in the eye is cut in half approximately every week.
However, the clinical effect, which is what truly matters to the patient, lasts significantly longer than the drug’s half-life. The therapeutic effect persists because the drug only needs to maintain a concentration above a certain threshold to continue blocking the VEGF protein. Even as the concentration drops, the remaining medication continues to neutralize the target protein, stabilizing the macula.
For most patients, the clinical benefit of a single injection—meaning the period of vessel stabilization and reduced fluid—is generally observed to last for about four to eight weeks. This duration represents the time before the abnormal blood vessels become active again and fluid begins to reaccumulate. Factors like prior eye surgery, such as a vitrectomy, can influence the precise rate of drug clearance, potentially requiring a shorter interval between treatments.
Determining the Retreatment Schedule
Maintaining a dry macula and stable vision requires a highly individualized retreatment schedule based on the drug’s effective duration. Ophthalmologists use diagnostic tools like Optical Coherence Tomography (OCT) scans to measure fluid levels and visual acuity tests to assess the therapeutic response. Fluid recurrence shown on an OCT scan signals that the previous dose’s clinical effect has worn off, indicating the need for another injection.
Retreatment Strategies
Doctors use three common strategies to manage injection frequency:
- Fixed Dosing Schedule: Injections are given at regular, predetermined intervals, often monthly, regardless of immediate clinical signs.
- Pro Re Nata (PRN) Regimen: The patient is monitored frequently, and an injection is administered only if the disease shows signs of recurrence. This minimizes the number of injections but requires close monitoring.
- Treat-and-Extend (T&E) Protocol: Once the macula is stable, the interval between injections is gradually extended (typically by one or two weeks), with an injection given at every visit. The interval is extended until recurrence is detected, then shortened slightly to maintain control.
Localized Treatment vs. Systemic Absorption
The medication is specifically injected into the eye because the target tissue is local, but trace amounts of Avastin do enter the general bloodstream, a process known as systemic absorption. The medication is an antibody, which eventually clears from the eye into the body’s circulation.
The amount of Avastin used for an intravitreal injection is extremely small, typically 1.25 milligrams, which is hundreds of times less than the dose given when the medication is used intravenously for other medical conditions like cancer treatment.
While the drug is detectable in the serum for several weeks after the eye injection, the concentration levels are very low. The drug clears from the blood relatively quickly, but its presence in the systemic circulation is generally not considered a major clinical concern due to the minimal dosage. The treatment is fundamentally a localized therapy, and the amount that spreads to the rest of the body is minimal.