Investigation of sterile hydrogels as topical vehicles for APOSEC™, a stressed peripheral blood mononuclear cell secretome for the treatment of poorly healing wounds

This study developed and evaluated a sterile, autoclavable hydrogel (APOgel) for the topical delivery of APOSEC™, demonstrating that while terminal sterilization enhanced drug release, the current syringe-mixing process requires optimization to ensure uniform distribution of active ingredients despite maintaining overall wound-healing efficacy.

The Gist

The Big Picture: Fixing Stubborn Wounds with "Liquid Gold"

Imagine you have a cut that just won't heal, no matter what you do. This is a common problem for people with diabetes. Doctors have a special "magic potion" called APOSEC™. It's not made of chemicals, but of tiny, healing signals (proteins and lipids) harvested from human blood cells that have been put under a little bit of stress to wake them up.

However, there's a catch: You can't just pour this liquid potion directly onto a deep, open wound. It would run off immediately. You need a carrier—a thick, sticky gel to hold the potion in place and let it soak in slowly.

This paper is about inventing a new, better "delivery truck" for this magic potion.

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The Problem with the Old Truck

In the past, hospital staff had to do a complicated dance to get the medicine ready:

1. They took a small vial of the liquid potion.
2. They took a separate syringe filled with a commercial gel (like a thick toothpaste).
3. They had to manually push the liquid back and forth between two syringes to mix them together.
4. Finally, they applied the mix to the wound.

The issues:

• Messy & Risky: Doing this manually in a hospital increases the risk of germs getting in.
• Inconsistent: If you mix it too hard, it gets too runny. If you don't mix it enough, the medicine isn't spread out evenly.
• One-Size-Fits-All: The old system was hard to adjust for small cuts vs. large burns.

The New Solution: The "Pre-Made" Gel

The researchers wanted to build a better system, which they called APOSEC 2.0. Their idea was simple:

• Pre-fill the gel: Instead of filling the syringe at the hospital, they wanted to put the sterile gel into the syringe at the factory.
• Sterilize it: They wanted to cook the gel in a high-pressure steam oven (autoclave) to kill any germs, just like you sterilize baby bottles.
• Mix on demand: The doctor would just add the liquid potion to the pre-filled syringe, give it a quick shake, and apply it.

The Experiment: Did it Work?

The team had to test three main things:

1. The "Cooking" Test (Sterilization)

The Analogy: Imagine making a delicate Jell-O. If you boil it, it might turn into soup.
The Result: They had to make a special gel recipe (called APOgel) that could survive the high heat of the steam oven without turning into soup. They tweaked the recipe (adding more thickening agents) so that after the "cooking," the gel was still the right thickness—just like the old commercial gel.

• Verdict: ✅ Success! The gel survived the heat and stayed thick enough to hold the medicine.

2. The "Shaking" Test (Mixing)

The Analogy: Imagine you have a bottle of thick honey and you pour a shot of water into it. If you shake it, does the water spread out evenly, or does it stay in clumps?
The Result: They mixed the liquid potion with the new gel by pushing it back and forth between syringes 20 times.

• The Surprise: They found that the mixture wasn't perfectly even. The first bit of gel squeezed out had a little more medicine, the middle had a little less, and the last bit had a little more again. It was like squeezing a toothpaste tube where the flavor isn't distributed perfectly.
• Verdict: ⚠️ Needs Improvement. The mixing method needs to be tweaked to ensure every drop has the exact same amount of medicine.

3. The "Healing" Test (Does it work on animals?)

The Analogy: Does the new delivery truck actually get the medicine to the destination and help the wound heal?
The Result: They tested this on mice with open wounds. They compared the new gel (APOgel) against the old commercial gel (Nu-Gel).

• The Twist: In the lab, the new gel let the medicine escape faster than the old one. You might think this is bad, but in wound healing, sometimes you want the medicine to release quickly to jumpstart the healing process.
• The Outcome: Despite the different speeds, both gels healed the wounds at the exact same rate. The new gel worked just as well as the expensive, old one.
• Verdict: ✅ Success! The new gel is just as effective as the old one.

The Takeaway

This paper is a success story for innovation. The researchers built a new, factory-sterilized gel that is safer and easier to use than the old manual mixing method.

• What's Great: The new gel is sterile, stable, and heals wounds just as well as the current standard.
• What Needs Work: The "shaking" method used to mix the liquid and gel needs a little fine-tuning to make sure the medicine is spread out perfectly evenly in every drop.

In short: They built a better delivery truck for a healing potion. It drives just as well as the old one, but the loading dock (the mixing process) needs a few more adjustments to be perfect.

Technical Summary

1. Problem Statement

Chronic, non-healing wounds (e.g., diabetic foot ulcers) are a significant clinical burden. APOSEC™, a cell-free secretome derived from irradiated peripheral blood mononuclear cells (PBMCs), has shown promise in regenerating tissue. However, its current clinical administration involves a cumbersome, manual, and potentially contamination-prone process:

• Current Workflow: Hospital staff must manually reconstitute lyophilized APOSEC™ powder, aspirate it into a syringe, connect it to a second syringe containing a commercial hydrogel (Nu-Gel), mix them manually, and apply the mixture to the wound.
• Limitations: This multi-step process increases preparation time, risks microbial contamination, and lacks flexibility for dosing different wound sizes (currently limited to a single fixed dose).
• Goal: Develop a "APOSEC 2.0" system featuring a terminally sterilized, pre-filled hydrogel (APOgel) in a syringe that can be mixed with the liquid APOSEC™ via a closed-system syringe-to-syringe method, allowing for multi-dose dispensing while maintaining sterility and efficacy.

2. Methodology

The study was divided into three main phases: formulation development, in vitro characterization, and in vivo efficacy testing.

A. Formulation Development (APOgel)

• Objective: Create a hydrogel compatible with terminal steam sterilization (autoclaving at 121°C, 15 min) that mimics the commercial benchmark, Nu-Gel (an alginate-based gel).
• Composition: Based on Nu-Gel but modified to withstand heat. The key modification was increasing Hydroxyethyl Cellulose (HEC) from 0.1% to 2.0% w/w to compensate for viscosity loss during sterilization.
• Process: The gel was produced in a GMP environment, filled into Cyclic Olefin Polymer (COP) syringes (3g), and terminally sterilized.
• Mixing Protocol: A closed-system mixing method was tested where 1 mL of liquid (APOSEC™ surrogate: AM2 medium) is mixed with 3 g of APOgel by passing the contents back and forth between two syringes 20 times via a Luer connector.

B. In Vitro Characterization

• Rheology: Dynamic viscosity was measured (1–100 s⁻¹ shear rates) to ensure the post-sterilized gel matched Nu-Gel and that the final mixture was suitable for dispensing.
• Sterility & Stability:
  • Sterility: Tested against Bacillus subtilis and Aspergillus brasiliensis to ensure no microbial growth and no inhibitory effects on the test organisms.
  • Chemical Stability: Analyzed via ATR-FTIR to detect degradation products and pH measurements to check for acidic shifts.
• Mixing Uniformity:
  • Viscosity Uniformity: Tested across different operators to assess reproducibility.
  • Content Uniformity: Used Sodium Fluorescein (SF) (small molecule) and Total Protein (large molecule) as surrogates. Three sequential 1 mL doses were dispensed from the mixed syringe to check for concentration gradients.
• Release Kinetics: Franz diffusion cells were used to measure the release of SF and proteins over 6 hours (high gradient) and 72 hours (therapeutic timeframe). Swelling and evaporation were monitored via gravimetry.

C. In Vivo Efficacy

• Model: Full-thickness excisional wounds (1 × 1 cm) on female Balb/C mice.
• Treatment: Mice received topical applications of APOSEC™ mixed with either APOgel or Nu-Gel every 48 hours for 10 days.
• Analysis: Wound area was quantified daily using ImageJ. Statistical analysis was performed using two-way repeated-measures ANOVA.

3. Key Contributions

• Development of an Autoclavable Hydrogel: Successfully formulated a hydrogel (APOgel) that survives terminal sterilization without significant chemical degradation, maintaining a viscosity profile comparable to the commercial benchmark.
• Closed-System Mixing Validation: Validated a syringe-to-syringe mixing protocol that reduces preparation time and contamination risk, though it highlighted specific challenges with homogeneity.
• Mechanistic Insight into Release: Demonstrated that the sterilization process alters the gel network, leading to increased swelling and faster release kinetics of active agents compared to the benchmark.
• In Vivo Equivalence: Provided critical data showing that despite differences in in vitro release profiles, the new formulation performs equivalently to the established benchmark in a living system.

4. Key Results

Formulation & Physical Properties

• Viscosity: Post-sterilization, APOgel viscosity (~325–350 Pa·s) was comparable to Nu-Gel. The increase in HEC successfully compensated for heat-induced viscosity loss.
• Stability: ATR-FTIR showed no new degradation peaks post-autoclaving. pH remained stable (approx. 8.5–8.7), with no significant acidic shift.
• Sterility: The sterilized gel passed sterility tests (no microbial growth after 15 days) and did not inhibit bacterial growth in validation tests.

Mixing & Dispensing Performance

• Viscosity Reduction: Mixing the liquid with the gel reduced viscosity by ~67% (APOgel) and ~56% (Nu-Gel), making it easier to dispense.
• Operator Variability: The mixing procedure was highly reproducible across different operators (Coefficient of Variation < 6%).
• Content Uniformity (Critical Finding): The study revealed non-homogenous distribution of active ingredients within the syringe. The first and last 1 mL doses contained ~20% more active ingredient (SF and proteins) than the middle dose. This indicates a concentration gradient along the syringe barrel, likely due to fluid dynamics during the mixing process.

Release Kinetics

• Faster Release: APOgel mixtures showed 32–48% higher release of small molecules and total proteins over 72 hours compared to Nu-Gel.
• Swelling: APOgel exhibited significant swelling (mass increase) in the donor chamber, whereas Nu-Gel showed minimal swelling. This suggests the sterilized gel network is more porous, facilitating faster diffusion and potentially better exudate uptake.

In Vivo Efficacy

• Wound Healing: Both APOgel and Nu-Gel formulations promoted wound healing significantly over time.
• Statistical Equivalence: There was no significant difference in wound closure kinetics between the APOgel and Nu-Gel groups (p = 0.94).
• Conclusion: Despite the in vitro differences (faster release, higher swelling), the in vivo therapeutic outcome was identical.

5. Significance and Implications

• Clinical Translation: The study proves that a terminally sterilized, pre-filled hydrogel is a viable alternative to manual compounding for biological wound therapies. This improves sterility assurance and workflow efficiency in hospital settings.
• Device Optimization Needed: The finding of non-uniform drug distribution in the syringe is a critical quality attribute (CQA) failure for the current mixing design. It suggests that for multi-dose dispensing, the mixing geometry or protocol must be optimized to ensure dose accuracy, or the product should be designed as a single-dose unit.
• Release vs. Efficacy: The disconnect between in vitro release rates and in vivo efficacy highlights that faster release does not necessarily equate to better healing in complex biological environments. The APOgel's ability to swell and absorb exudate may be a beneficial mechanism not captured by simple release models.
• Broader Impact: The methodology and findings are applicable to the emerging field of Extracellular Vesicle (EV) therapeutics, where co-formulation with gels for localized delivery is increasingly common but technically challenging regarding mixing homogeneity and stability.

In summary, the paper successfully develops a sterile, autoclavable hydrogel platform for APOSEC™ that matches the clinical efficacy of the current standard but requires further engineering to ensure uniform drug distribution during the mixing and dispensing process.