Affibody molecules are a class of engineered proteins designed to bind to specific molecular targets within the body. These small proteins act with high precision, similar to how a unique key fits only one lock. They are developed to attach to certain proteins, including those on the surface of cancer cells or other markers of disease. This binding capability allows them to be used for various medical purposes, which are determined by their specific design and the molecules they are engineered to target.
The Affibody Scaffold and Design
Affibody molecules are built using a structural framework, or scaffold, derived from a protein found on the surface of the bacterium Staphylococcus aureus. This source protein, called Protein A, has five domains that bind to antibodies. Scientists took one of these, the B-domain, and modified it to enhance its stability, creating what is now known as the Z-domain. This Z-domain serves as the foundational scaffold for all Affibody molecules and consists of 58 amino acids arranged into a compact three-helix bundle.
The design process involves altering specific amino acids on the surface of this Z-domain scaffold to grant it a new binding function. Out of the 58 amino acids, 13 are located on the first two helices and are accessible to the surrounding environment. By changing these 13 residues through a process of random mutation, a vast library of different Affibody variants can be generated. This library is then screened against a desired target molecule, such as a protein associated with a particular disease, to identify variants that bind with high affinity and specificity.
Distinctions from Monoclonal Antibodies
A primary distinction between Affibody molecules and conventional monoclonal antibodies (mAbs) is their substantial difference in size. Affibody molecules are significantly smaller, with a molecular weight of about 6.5 kilodaltons (kDa), compared to mAbs, which are approximately 150 kDa. This smaller size allows Affibody molecules to move more effectively into dense tissues, such as solid tumors, where the larger size of mAbs can limit penetration and distribution.
The production methods for these two types of molecules also differ considerably. Affibody molecules are produced in bacterial systems, such as E. coli, which is a rapid and cost-effective process. In contrast, mAbs require complex and expensive manufacturing processes using mammalian cell cultures.
Another point of contrast is their stability. The three-helix bundle scaffold of an Affibody is very stable, both chemically and thermally. Their small size also dictates how they are processed in the body. Affibody molecules are cleared from the bloodstream quickly through the kidneys, resulting in a short half-life.
Mechanisms in Medical Imaging
The properties of Affibody molecules make them well-suited for use as agents in medical imaging. For diagnostic purposes, an Affibody that is engineered to bind to a specific disease marker is chemically linked to a radioactive isotope. This process, known as radiolabeling, creates a probe that can be detected by specialized imaging equipment.
After being administered to a patient, the radiolabeled Affibody circulates throughout the body. Due to its engineered specificity, it accumulates at sites where its target is present, such as a tumor. Any unbound Affibody molecules are quickly filtered out of the bloodstream and cleared by the kidneys.
This rapid clearance from non-target tissues and the bloodstream is advantageous for imaging. It leads to a high-contrast image where the signal from the targeted tissue is strong and the background noise from unbound molecules is low. This allows for clear visualization of target sites using techniques like Positron Emission Tomography (PET) or Single-Photon Emission Computed Tomography (SPECT). For example, an anti-HER2 Affibody is used in imaging for patients with breast cancer.
Therapeutic Roles and Development
Beyond diagnostics, Affibody molecules are being developed for therapeutic applications. One primary therapeutic strategy involves using an Affibody to block the function of a harmful protein. By binding tightly to a specific site on a target protein, the Affibody can inhibit its activity, which may be driving a disease process.
Another therapeutic approach uses Affibody molecules for targeted drug delivery. In this strategy, a potent cytotoxic agent, such as a chemotherapy drug or a toxin, is attached to an Affibody. The Affibody then acts as a guiding system, delivering the toxic payload directly to cells that display its specific target, for example, cancer cells.
Research is ongoing to expand their use against various diseases. For instance, Affibody molecules have been designed to target proteins involved in immune responses, such as PD-L1, which is a target in cancer immunotherapy. These developments highlight the adaptability of the Affibody platform for creating targeted treatments.