Is TB4-Frag (Ac-SDKP) the same as TB-500?

No. Both are fragments of Thymosin Beta-4, but they are different sequences with different functions. TB-500 is the actin-binding fragment (LKKTETQ), used for tissue repair and cell migration. Ac-SDKP (TB4-Frag) is the N-terminal tetrapeptide (N-acetyl-Ser-Asp-Lys-Pro), used for anti-fibrotic organ protection — cardiac, renal, and pulmonary fibrosis prevention. They share a parent protein but have distinct mechanisms, applications, and evidence bases.

How does Ac-SDKP relate to ACE inhibitors?

Ac-SDKP is a natural substrate of ACE (angiotensin-converting enzyme), which normally degrades it. ACE inhibitors (lisinopril, enalapril, ramipril, and others) block this breakdown, raising endogenous Ac-SDKP levels roughly 3–5 fold. Some of the organ-protective benefits associated with ACE inhibitors — particularly cardiac and renal antifibrotic effects — may be mediated in part through this elevation of endogenous Ac-SDKP.

COMPOUND LIBRARY·TB4-FRAG (AC-SDKP)

A DIFFERENT THYMOSIN BETA-4 FRAGMENT — NOT TB-500

TB-500 is the actin-binding fragment (LKKTETQ) of Thymosin Beta-4 — built for tissue repair and cell migration. Ac-SDKP (TB4-Frag) is a different fragment entirely — the N-terminal tetrapeptide (N-acetyl-Ser-Asp-Lys-Pro) — built for anti-fibrotic organ protection. Different sequence, different mechanism, different application. Read the TB-500 page → · Read the full Thymosin Beta-4 page →

COMPOUND PROFILE · PEPPERLEDGER

TB4-Frag / Ac-SDKP

N-Acetyl-Seryl-Aspartyl-Lysyl-Proline (N-acetyl-Ser-Asp-Lys-Pro) · Endogenous — naturally present in human plasma at ~0.5 nM

Type

Endogenous tetrapeptide — derived from Thymosin Beta-4 by enzymatic cleavage (prolyl oligopeptidase) and naturally circulating in plasma

Class

Anti-fibrotic · Organ-protective · TGF-β1 pathway inhibitor · Stem cell mobilizer · Anti-inflammatory · Natural ACE-inhibitor substrate

Administration

Subcutaneous injection · IV (research) · Oral (poor bioavailability)

Half-life

Short — naturally degraded by ACE; ACE inhibitors elevate endogenous levels 3–5x by blocking this breakdown

Most studied use

Cardiac fibrosis prevention · Renal protection · Pulmonary fibrosis · Hematopoietic stem cell mobilization · Anti-fibrotic organ protection

Regulatory status

Not FDA-approved · Research compound · Human studies exist for cardiac and renal protection · Not a biohacker-mainstream compound — more of a clinical research molecule with growing awareness

Human evidence

Moderate — human studies in cardiac fibrosis and renal protection (often observed indirectly via ACE inhibitor effects); limited dedicated exogenous-dosing human trials

Preclinical evidence

Strong — anti-fibrotic effects confirmed across cardiac, renal, pulmonary, and hepatic fibrosis models; mechanism well-characterized

EDUCATIONAL TOOL — NOT MEDICAL ADVICE

What is TB4-Frag (Ac-SDKP)?

Ac-SDKP (N-acetyl-Ser-Asp-Lys-Pro) is a naturally occurring tetrapeptide that most people in the TB-500 and Thymosin Beta-4 community haven't heard of — despite being derived from the same parent protein and carrying its own substantial evidence base. Where TB-500 targets tissue repair and cell migration through the actin-binding LKKTETQ sequence, Ac-SDKP has a completely different primary application: anti-fibrosis and organ protection.

Fibrosis — the accumulation of scar tissue in organs — is a leading driver of organ failure and age-related organ dysfunction. Cardiac fibrosis impairs heart function. Renal fibrosis drives chronic kidney disease. Pulmonary fibrosis is often fatal. Hepatic fibrosis drives cirrhosis. Current anti-fibrotic treatment options are limited, and Ac-SDKP is one of a small number of endogenous molecules with demonstrated anti-fibrotic activity across multiple organ systems.

The mechanism is well-characterized: Ac-SDKP inhibits TGF-β1-driven fibroblast activation and collagen synthesis — the primary pathway through which fibrosis develops — and reduces inflammatory cytokine production in macrophages. There's also an important clinical connection: ACE inhibitors (the widely prescribed blood pressure medications) work partly by blocking the enzyme that degrades Ac-SDKP, raising endogenous Ac-SDKP levels 3–5 fold. Some of ACE inhibitors' well-documented organ-protective benefits may be mediated through this pathway.

How it works

TGF-β1 Pathway Inhibition — The Anti-Fibrotic Core

TGF-β1 (transforming growth factor beta 1) is the primary driver of fibrosis in every organ. It signals through Smad2/3 to activate fibroblast proliferation, myofibroblast differentiation, and collagen type I/III synthesis — the steps that produce pathological scar tissue. Ac-SDKP interferes with TGF-β1 signaling, reducing Smad phosphorylation and downstream fibroblast activation. This has been confirmed in cardiac, renal, pulmonary, and hepatic fibrosis models.

Macrophage and Inflammatory Inhibition

Ac-SDKP inhibits macrophage activation toward pro-inflammatory phenotypes, reducing TNF-α, IL-1β, and MCP-1 production — the cytokines that drive inflammatory fibrosis. By dampening the inflammatory trigger that precedes fibrotic signaling (macrophage activation → TGF-β1 → fibroblast activation), Ac-SDKP intervenes earlier in the cascade than compounds that directly block TGF-β1 alone.

Hematopoietic Stem Cell Biology

Ac-SDKP was originally identified as an inhibitor of hematopoietic stem cell (HSC) proliferation — keeping HSCs quiescent in G0 phase and protecting them from chemotherapy damage. In other contexts, it also mobilizes HSCs from bone marrow into peripheral blood. This dual HSC behavior is part of why Ac-SDKP has been studied in cancer supportive-care settings.

The ACE Inhibitor Connection

Ac-SDKP is a natural substrate of ACE (angiotensin-converting enzyme) — ACE normally degrades it. When ACE inhibitors (lisinopril, enalapril, ramipril) block ACE, Ac-SDKP plasma levels rise 3–5 fold. This is thought to contribute to ACE inhibitors' organ-protective effects beyond blood pressure reduction — a meaningful share of the cardiac and renal benefit attributed to this drug class may be mediated through endogenous Ac-SDKP elevation.

What the research shows

STUDYCirculation · 2003

Ac-SDKP prevents and reverses cardiac fibrosis in hypertensive rats — and is elevated by ACE inhibitor use in humans

Peng H et al.

Hypertensive rat model: Ac-SDKP administration prevented and partially reversed cardiac fibrosis, reduced collagen deposition, and improved cardiac function. Companion human observation: patients on ACE inhibitors (with elevated endogenous Ac-SDKP) showed less cardiac fibrosis than controls. Establishes both the cardiac fibrosis application and the ACE inhibitor mechanism connection.

View on PubMed →

STUDYJournal of the American Society of Nephrology · 2006

N-acetyl-seryl-aspartyl-lysyl-proline prevents renal fibrosis

Omata M et al.

Renal fibrosis model. Ac-SDKP significantly reduced tubulointerstitial fibrosis and macrophage infiltration, and preserved renal function. Establishes the renal-protective application and confirms the anti-inflammatory/anti-fibrotic mechanism in kidney tissue specifically.

View on PubMed →

STUDYHypertension · 2007

Role of Ac-SDKP in the antifibrotic and anti-inflammatory effects of the ACE inhibitor captopril in cardiac hypertrophy

Peng H, Carretero OA, Liao TD et al.

Demonstrates that a meaningful share of captopril's (an ACE inhibitor's) antifibrotic and anti-inflammatory cardiac benefit is mediated through elevated endogenous Ac-SDKP — directly linking the peptide's biology to a drug class already used in millions of patients.

View on PubMed →

Common misconceptions

"TB4-Frag is the same as TB-500."

Reality: Both are fragments of Thymosin Beta-4, but they are completely different sequences with different primary functions. TB-500 = LKKTETQ (the actin-binding fragment) → tissue repair, cell migration, wound healing. Ac-SDKP = N-acetyl-SDKP (the N-terminal fragment) → anti-fibrotic, organ protection, stem cell biology. Different compounds, different mechanisms, different evidence bases — sharing only a parent protein.

"Taking an ACE inhibitor is the same as supplementing Ac-SDKP."

Reality: ACE inhibitors raise endogenous Ac-SDKP 3–5 fold by blocking its breakdown — but exogenous supplementation changes plasma levels through a different pharmacokinetic route, and ACE inhibitors carry their own independent effects (blood pressure reduction, direct renal protection, bradykinin elevation) that confound any direct comparison. For someone already on an ACE inhibitor, exogenous Ac-SDKP would add to an already-elevated baseline.

"Ac-SDKP reverses existing fibrosis."

Reality:The preclinical data shows Ac-SDKP prevents fibrosis progression, and in some models produces partial reversal of established fibrosis — but complete reversal of established scarring is not shown for this or any compound; fibrosis reversal is mechanistically difficult. Ac-SDKP's strongest application is prevention and slowing progression, not reversing existing damage.

Community knowledge

Ac-SDKP / TB4-Frag has a small but growing community — primarily people who have researched Thymosin Beta-4's individual fragments in depth, and those specifically interested in cardiac and renal protection. It's far less known than TB-500, but is gaining attention as the fragment literature becomes better understood. Common reports: it's used for organ protection rather than acute repair (a different application than TB-500 entirely); it's often stacked with TB-500 — TB-500 for acute tissue repair and cell migration, Ac-SDKP for longer-term organ-protective effects, the two viewed as complementary; people with hypertension, prior cardiac events, or significant cardiovascular risk factors specifically seek it out for the cardiac anti-fibrotic mechanism; and — because organ protection is a slow, preventive process — users don't report the dramatic subjective effects associated with TB-500 or BPC-157. This tends to be a research-oriented community that arrived here by reading the mechanism literature, not by following trends.

WHAT THE RESEARCH SHOWS

✓KNOWN

✓TGF-β1 pathway inhibition produces anti-fibrotic effects across cardiac, renal, pulmonary, and hepatic models
✓ACE inhibitors elevate endogenous Ac-SDKP 3–5 fold — a well-established mechanistic connection to a drug class used in millions of patients
✓Human data exists showing reduced cardiac fibrosis associated with elevated Ac-SDKP (via ACE inhibitor use)
✓Reduces macrophage-driven inflammatory cytokine production (TNF-α, IL-1β, MCP-1) — addressing fibrosis upstream
✓Mechanistically distinct from and complementary to TB-500 — different fragment, different sequence, different application

?UNCERTAIN

?Optimal exogenous dosing for biohacker / organ-protection applications — no established human protocol exists
?Long-term safety of exogenous administration — most human data comes from indirect ACE-inhibitor elevation, not direct dosing
?Whether exogenous Ac-SDKP provides meaningful additional benefit for users not already on ACE inhibitors
?Whether it can reverse — rather than only slow — established organ fibrosis in humans

RECONSTITUTION CALCULATOR

Peptide in vial (mg)BAC water (mL)Desired dose (mcg)

EDUCATIONAL TOOL — NOT MEDICAL ADVICE · VERIFY WITH YOUR CLINICIAN

AI COACH PREVIEW

I have hypertension and want to prevent cardiac fibrosis. How should I think about Ac-SDKP alongside my current protocol?

Ac-SDKP is mechanistically well-matched to that goal — its primary anti-fibrotic application is exactly cardiac and renal protection, and there's a direct clinical connection worth understanding first: if you're already on an ACE inhibitor (lisinopril, enalapril, ramipril, etc.) for your hypertension, your endogenous Ac-SDKP is already elevated 3–5 fold, because ACE inhibitors work partly by blocking the enzyme that breaks Ac-SDKP down. That's not a reason to skip exogenous Ac-SDKP — it's a reason to go in informed and to loop in your prescribing physician, since you'd be layering an exogenous source on top of an already-elevated baseline. Community-derived starting point (no formal human biohacker protocol exists): 100–500 mcg subcutaneous, three times weekly, run long-term or in 12-week cycles with monitoring — organ protection is a slow, sustained-effect goal, not an acute one. Practically, this stacks naturally with TB-500: TB-500 covers acute tissue repair and cell migration, Ac-SDKP covers the longer-horizon anti-fibrotic organ-protective layer — together they're a more complete tissue-maintenance approach than either alone. What I'd track in PepperLedger: dose and timing for every injection, blood pressure monthly, kidney function (creatinine, eGFR) periodically, an echocardiogram at baseline and annually if cardiac fibrosis prevention is the central goal, and inflammatory markers (hs-CRP, TNF-α) if you have access to them. Bring your ACE inhibitor regimen into the conversation with your cardiologist before starting — that's the one piece of this that genuinely needs a clinician's input.

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