503A Compounding

503A

Traditional pharmacy compounding under section 503A of the Federal Food, Drug, and Cosmetic Act. Requires a patient-specific prescription and is not subject to FDA pre-approval. Quality varies by pharmacy.

503B Outsourcing Facility

503B

FDA-registered outsourcing facility under section 503B. Subject to cGMP (current good manufacturing practice) standards and FDA inspection. Higher quality bar than 503A.

Adverse Event

AE, TEAE, Treatment-Emergent Adverse Event

Any unfavorable medical occurrence in a study participant during the trial — regardless of whether it is causally related to the treatment. TEAE specifically refers to events occurring after treatment begins.

Affinity

Binding Affinity, Ki, Kd

How tightly a compound binds to its target receptor. Higher affinity means the compound holds on more strongly and can produce effects at lower concentrations. Often reported as Ki or Kd, where lower numbers mean tighter binding.

Albumin Binding

The attachment of a peptide to albumin (the most abundant protein in blood plasma). Albumin binding shields the peptide from rapid kidney clearance, extending its half-life. Many long-acting peptide drugs are deliberately engineered with a fatty-acid tail that anchors them to albumin — semaglutide, cagrilintide, and eloralintide all use this strategy.

Aliquot

A measured portion of a larger sample. Used as both a noun ('an aliquot of solution') and a verb ('to aliquot a vial'). After reconstitution, peptides are sometimes split into smaller aliquots to limit how many times each vial is punctured, reducing contamination risk.

Allosteric Modulator

Positive Allosteric Modulator, Negative Allosteric Modulator, PAM, NAM

A compound that binds to a site on a receptor distinct from where the receptor's natural ligand binds. Doesn't activate the receptor on its own — it changes how the receptor responds to its natural signal. Positive allosteric modulators amplify the response; negative ones dampen it.

Amidation

C-terminal Amidation

A modification of the peptide's C-terminus that replaces the carboxyl group with an amide. Improves stability and is required for activity in many native peptides.

Amino Acid

The molecular building blocks of peptides and proteins. Twenty standard amino acids occur in human proteins; synthetic peptides can include non-standard residues for stability or activity.

Amylin Receptor Agonist

Amylin RA

Activates amylin receptors (AMY1R, AMY2R, AMY3R) found in the brainstem and hypothalamus. Drives satiety, slows gastric emptying, and reduces postprandial glucose. Examples: pramlintide, cagrilintide, eloralintide.

Analog

Peptide Analog

A modified version of a natural peptide, engineered to improve stability, potency, selectivity, or duration of action. Semaglutide is a GLP-1 analog of native GLP-1; cagrilintide is an analog of native amylin.

AUC

Area Under the Curve

Total drug exposure over time, calculated as the area under the plasma concentration-time curve. Primary measure of overall exposure and the basis for dose-proportionality assessments.

Avoid Combination

The compounds should not be used together. Reasons include redundant mechanisms providing no additional benefit, unacceptable additive risk, or documented adverse interactions.

Bacteriostatic Water

BAC Water

Sterile water containing 0.9% benzyl alcohol as a preservative. The most common diluent for peptide reconstitution because the preservative inhibits microbial growth, allowing multi-dose use over roughly 28 days when refrigerated.

BDNF

Brain-Derived Neurotrophic Factor

A protein that supports the growth, survival, and connectivity of neurons. Many nootropic peptides (semax, P21, cerebrolysin, dihexa) are valued for raising BDNF, since higher BDNF correlates with improved learning, memory, and resistance to neurodegeneration.

Bioavailability

The fraction of an administered dose that reaches systemic circulation, expressed as a percentage. IV is 100% by definition; oral peptide bioavailability is typically under 1% without special formulation.

BLA

Biologics License Application

A Biologics License Application — the regulatory pathway for biologic products (including most peptides and proteins). Functionally similar to an NDA but used for biologic drugs.

Bolus

A single, defined dose delivered all at once rather than spread over time — the opposite of an infusion. Most peptide injections are bolus doses.

Clearance

The rate at which the body removes a compound from the bloodstream, expressed in volume per time (e.g., mL/min). Combined with Vd, clearance determines half-life.

Cmax

Peak Concentration

The maximum (peak) plasma concentration reached after dosing. Used alongside Tmax to characterize absorption profile.

Compatible

Two compounds can be safely used together with no clinically meaningful interaction. Combined activity equals the sum of their individual effects (1+1=2). No enhancement, no antagonism.

Compounded

A drug prepared by a pharmacy to meet a specific patient's needs, rather than mass-manufactured by the original innovator. Compounded peptides are not FDA-approved and quality controls vary widely.

Contraindication

Absolute Contraindication, Relative Contraindication

A specific reason a compound should NOT be used. Absolute contraindications (known allergy, certain cancers, pregnancy for some compounds) make the compound off-limits. Relative contraindications mean use with caution and extra monitoring. Listed on every FDA-approved drug label and in most clinical trial exclusion criteria.

Cycle

Cycling, On-Cycle, Off-Cycle

An on-then-off period of compound use — e.g., 'eight weeks on, four weeks off.' Cycling is used to prevent receptor desensitization, give the body recovery time, and limit cumulative exposure. Common in growth-hormone secretagogue protocols; less common with GLP-1 agonists, which are typically dosed continuously.

Cyclic Peptide

Cyclic Structure

A peptide whose chain forms a closed ring rather than a linear strand — typically via a bond between the two ends or a disulfide bridge between internal residues. Cyclic peptides are usually more resistant to enzymatic breakdown than linear ones, which can mean longer half-lives and sometimes oral bioavailability.

Diluent

Solvent

Any sterile liquid used to dissolve a lyophilized peptide into a usable injection. The three most common are bacteriostatic water, sterile water for injection, and 0.9% sodium chloride (saline). Each has different shelf-life and storage implications once the vial is mixed.

Disulfide Bridge

Disulfide Bond

A covalent bond formed between two cysteine residues, creating loops and stabilizing peptide structure. Sometimes replaced with thioether or methylene bridges to improve stability.

Dose Dependent

Safety and efficacy of the combination depend on dose. Low doses may be compatible while higher doses produce additive side effects or unintended interactions.