Blood-brain barrier permeable peptide compositions

A method for assessing a target biomarker in the central nervous system of a subject, the method comprising:

• (i) administering to the subject a polypeptide comprising the formula:
• Vab-Z-Z′
• wherein Vab comprises all or a portion of a variable antigen-binding (Vab) domain of a camelid or shark single domain heavy chain antibody,
• wherein Z comprises all or a portion of a hinge region from an IgG or a linker comprising —NHCO—,
• wherein Z′ comprises all or a portion of at least one IgG CH domain,
• wherein the polypeptide further comprises a detectable label, and
• wherein the polypeptide is capable of specifically binding to a target biomarker within the central nervous system (CNS) of the subject;
• (ii) waiting for a time sufficient for the polypeptide to permeate the blood-brain-barrier of the subject and bind to the target biomarker, if present;
• and
• (iii) determining the presence or amount of the detectable label within the CNS of the subject.