Drug Substance to Fill-Finish Handoff: What Sponsors Need to Know

The drug substance-to-fill-finish handoff is one of the most consequential transitions in a drug development program, and one that all parties are navigating together, often with an incomplete picture of the product.

For new or early-phase formulations, the fill/finish CDMO may be working from limited batch history, sometimes only pilot or engineering batches at much smaller scale. Sponsors, drug substance (DS) partners, and fill/finish CDMOs are all learning about a product as it moves through clinical development. That’s not a planning failure. It’s the nature of early-phase drug development. The knowledge base grows with the program.

What makes the handoff worth careful attention is that scale-up isn’t always linear. You might scale most excipients one amount for a commercial batch but find that one excipient needs to be scaled differently to achieve the same results. You don’t know that until you do it. Those variations tend to surface in the critical components – the excipients and API that matter most to product quality. Involving the fill/finish CDMO early isn’t just about catching problems before they become costly; it’s about building the product familiarity that makes every subsequent phase go more smoothly, and ultimately getting a safe, effective product to the patients who need it.

When drug substance and fill-finish are managed by separate organizations, the fill/finish CDMO is dependent on two parties it does not control: the DS manufacturer and the sponsor. Manufacturing schedules, material availability, and timeline commitments at the fill/finish CDMO are all built around when drug substances will arrive, which is decided by others.

• All three parties, sponsor, DS manufacturer, and fill/finish CDMO, need to work from the same timeline. A delay at the DS CDMO impacts fill/finish slot availability, stability program start dates, and time to market. When the fill/finish CDMO is the last to learn about a delay, it leaves the least time to adjust.
• Communication gaps. Informational risks, rather than technical, are the most difficult to manage in split arrangements. When important details about the product, batch, or timeline aren’t shared reliably, the fill/finish CDMO ends up making decisions based on incomplete information.
• Split arrangements function best when everyone has a realistic and shared understanding of what’s needed and when. When expectations about timelines, documentation, or material readiness aren’t aligned, it creates friction that slows down programs and puts batches at risk.

Early Planning for a Successful Fill-Finish Transfer

Programs that run smoothly usually involve a fill/finish CDMO that has time with the product before the pressure of later stages. There should be enough time to become familiar with the product, conduct development work, and build systems that will support the program in the long run. The more a fill/finish CDMO understands before GMP fill, the better the results in terms of quality, process improvement, and product longevity. Fluid-flow dynamics, agitation sensitivity, and the behavior of a product on equipment can be difficult to predict without hands-on experience with the actual product. As such, early clinical fills carry inherent risk and both sponsors and CDMOs should proceed with realistic expectations.

It’s worth acknowledging that having meaningful time with a product before cGMP filling is the ideal scenario, and one that isn’t always achievable. DS can be difficult and expensive to manufacture, meaning available material is limited and every gram is accounted for. Timelines are often compressed by clinical milestones, regulatory windows, or commercial pressure. Early-phase programs may not have the characterization data or process definition to support meaningful development work even when time exists. These are real constraints, not planning failures, and they’re more the rule than the exception. The goal isn’t to suggest that every program can or should follow the ideal path, it’s to help sponsors and CDMOs make the most of whatever window they have, and to recognize that every touchpoint with the product before cGMP work reduces risk for the patients ultimately depending on it. When product is available for development work ahead of cGMP fill, using it that way is almost always worth it.

The most impactful early actions sponsors can take:

• Engage the fill/finish CDMO well before the transfer.
• Share all the information about the product as early as possible, including what hasn’t been fully characterized.
• Build in development time wherever the program allows.

Consider what early involvement actually looks like. In early batches of a hypothetical product, a fill/finish CDMO discovers unexpected behavior during processing that manifests in a way that could indicate either a contamination issue or an inherent product characteristic. Determining which requires real investigation. Working collaboratively with the sponsor, the CDMO re-engineers the formulation process, closing up the formulation and implementing additional clarification steps prior to the subsequent addition of APIs and excipients, which resolves the final filtration challenge. This kind of collaborative problem-solving only happens when the fill/finish CDMO is engaged as a true partner.

Communication Across Three Parties

There’s no universal communication cadence that works for every program. A program filling twice a year has different needs than one in active commercial supply. The right cadence is the one that fits the program, and it’s up to the sponsor and CDMO to define this together.

What good communication looks like in three-party arrangements is a unified stance. When a problem surfaces, the goal is to find a solution together, not establish who is responsible. The sponsor sits at the center of the DS and fill/finish relationship and sets the tone for how all three parties work together. The more transparent all parties can be, the better the program runs and the better the outcomes for patients.

How MSAT Supports the Handoff

Manufacturing Science and Technology (MSAT) sits at the intersection of process design and manufacturing execution – exactly where handoff challenges tend to live. At Afton, MSAT’s role is to translate what’s known about a product into a system that operators can run reliably and safely, before cGMP manufacturing begins. Much of this work happens at the bench: studying how a product behaves under different conditions, designing the fill process around those properties, and resolving open questions before they become problems on the line. Once a system is designed, MSAT works directly with the production team to make the internal handoff as seamless as possible – building familiarity, standardizing processes across programs wherever practical, and ensuring operators understand not just what to do, but why. That shared understanding reduces deviation risk and makes the manufacturing environment safer for everyone involved, including the patients who ultimately receive the product.

Required Documents and Material Readiness

Receiving bulk drug substance is the moment when everything the fill/finish CDMO needs to do their job either arrives with the material or doesn’t. At minimum, the fill/finish CDMO needs to know what the material is, what condition it’s in, and whether it meets requirements for use. That means:

• Certification (Certificate of Analysis, Certificate of Compliance, Certificate of Processing, etc.) from the DS manufacturer
• Clear documentation of storage conditions and any excursions during transit
• Sub-packaging details
• Results of any testing completed by the DS manufacturer
• Risk assessments and container closure system documentation established ahead of receipt, if available

Phase 1 requirements differ from Phase 3 or commercial in meaningful ways. For good reason, early clinical programs require lighter documentation. As programs advance, requirements become more formal and comprehensive. The most effective way to ensure incoming requirements are met is to have an explicit conversation about them early.

Quality Ownership in Split CDMO Arrangements

One of the most important things for sponsors to understand about three-party DS/fill-finish arrangements is where quality governance really resides. In most cases there is no direct Quality Agreement between DS manufacturer and fill/finish CDMO. The relationships with the sponsor are one-to-one with each of the CDMOs. That means the sponsor is responsible for ensuring that its DS partner meets the requirements on which the fill/finish CDMO relies, and it needs to take that responsibility seriously.

The fill/finish CDMO cannot impose requirements on a DS manufacturer with whom they have no direct relationship. That’s the sponsor’s responsibility to manage. Managing two separate CDMO relationships is not only a coordination challenge, it’s a quality ownership challenge.

Managing a temperature-sensitive drug substance is about more than keeping it cold. It’s about having a deep enough understanding of the product’s physical properties to be able to design a system that protects it at every stage, from DS release to the start of the fill.

For example, if a hypothetical product freezes at room temperature, but that isn’t communicated to the fill/finish CDMO before manufacturing begins, it can cause significant problems during filling. Physical properties that seem incidental can be critical to operations.

Active monitoring becomes more critical once a shipment is in motion. Successful sponsors and CDMOs monitor in real time and know what to do when something goes wrong rather than finding out about a problem when the material arrives. The following questions should have answers before the shipment leaves:

• What is the protocol if there’s a significant delay?
• What if the shipment arrives outside the validated temperature range?
• How long can the product be out of condition, and at what point is material quarantined?

Contingency planning addresses what happens when something goes wrong. Pathway validation addresses how to reduce the chances they do. Validating the shipping method and pathway, not just the container, is a critical part of commercial supply planning. Understanding how shipping affects the product and working with a reputable logistics partner to scope the safest route from point A to point B, are steps that are easy to defer and costly to skip. At the end of the day, every decision in cold chain planning traces back to the patient. That framing should inform how seriously all parties approach this work.

• Understand your CDMO and set them up for success. Share all knowledge on the product early, including information still to be learned, and treat the CDMO as a partner.
• Have realistic timeline expectations. A product can be filled, inspected, and packaged, and still sit waiting for release testing results before it can ship. Sponsors need to understand the full process, from fill completion through release and shipment.
• Invest in your regulatory package. Shortcuts taken during development can resurface during regulatory review, potentially the worst possible time. They don’t disappear.
• Send product ahead of time. A CDMO that has handled your product before the first cGMP batch presents fundamentally lower risk than one learning about it in real time under manufacturing pressure.

The more a fill/finish CDMO knows about a product (its physical properties, its sensitivities, its history) the better equipped they will be to create systems that protect it and run it well. That familiarity has to be designed intentionally through early engagement, open dialogue and genuine partnership.