Antibody drug conjugate (ADC) therapies have developed rapidly in recent decades. Currently, 14 ADC drugs have been approved worldwide and more than 140 ADCs are in clinical trials. By 2030, the ADC market will reach more than $15 billion. The basic principle of ADCs: By combining the specificity of monoclonal antibodies with the cytotoxicity of potent small molecule drugs, ADCs can precisely deliver toxins to tumors.
However, although the FDA has approved multiple ADC drugs, the failure rate of ADCs during clinical development remains high. The inherent complexity of ADCs is a double-edged sword, providing opportunities to provide better treatment while also increasing confounding factors for treatment failure. ADC design drives its pharmacokinetics and pharmacodynamics, and requires more in-depth analysis than the commonly used Cmax and area under curve (AUC) metrics to obtain the optimal dose for clinical application. Current FDA-approved ADCs targeting solid tumors share some common characteristics, including the humanized IgG1 antibody domain, high expression of tumor receptors, and high doses of antibodies. These common features have potential implications for clinical pharmacokinetics and mechanisms of action and provide a reference for ADC design at all stages of clinical development.
3 Design Criteria Followed by Recently Approved ADCs
Three recently approved solid tumor ADCs highlights important design criteria. Although several components of ADCs show significant differences, their shared characteristics are noteworthy. The structures of the four FDA-approved ADCs for solid tumors are very different, including different linker types (cleavable vs. non-cleavable, different release mechanisms, different stability), specific and non-specific binding, different targets , cancer types and drug-to-antibody ratio (DAR). Interestingly, these ADCs share three common characteristics: (i) a high expression target (105–106 receptors/cells), (ii) a high antibody dose (3.6 mg/kg or greater over 3 weeks), and (iii) an IgG1 allotype antibody backbone.
These three common features have a significant impact on drug delivery and distribution. In fact, because ADCs use known cytotoxic payloads (such as microtubule inhibitors) and known targeting antibodies, a key feature of their clinical success is delivery -- payloads that target every tumor cell at a tolerated dose. These shared design features have their own implications for the targeted delivery of the payload to the tumor.
High Expression Target
HER2, Nectin-4, and Trop-2 are highly expressed tumor antigens with over 105 receptors per tumor cell and significantly reduced expression in healthy tissues. Because of the large expression differences, high expression targets can provide a larger window of treatment. Because drug delivery to healthy tissues is usually more effective than delivery to tumors, high antibody doses and high expression tumor targets can quickly saturate uptake in low-expression healthy tissues while still maximizing tumor uptake. The payload toxicity and/or DAR can then be modified to ensure delivery to tumor cells above the therapeutic threshold, while maintaining the subtherapeutic threshold in healthy tissue (to avoid target-mediated healthy tissue toxicity). In contrast, targeting lower expressed tumor antigens requires more potent payloads to achieve therapeutic concentrations in targeted cells. Increasing payload potency usually results in higher toxicity and a lower ADC-tolerated dose. While a lower ADC dose reduces tumor uptake, it may not reduce healthy tissue uptake by the same amount (for example, if target-mediated healthy tissue uptake remains saturated), potentially reducing the therapeutic index. It is important to note that this trade-off is very different from small-molecule drugs, which are usually balanced with plasma concentrations so that low doses result in lower exposure to healthy tissue. In contrast to small molecules, lower doses of more effective ADCs can limit tumor penetration, thereby reducing efficacy rather than toxicity.
High Antibody Dose
Developing ADCs for solid tumors is challenging because of poor vascular leakage, tortuosity, and poor lymphatic drainage in solid tumors, resulting in poor convection and increased interstitial pressure. These characteristics combine to create an unfavorable environment for the delivery of ADC drugs. The most direct way to increase antibody delivery and tissue penetration is to give higher doses of antibodies, a second common feature among currently approved ADCs.
Kadcyla is the first ADC to receive FDA approval for solid tumors in many years, and was approved in 2013 through the use of a human IgG1 skeleton, a medium DAR (3.5), a non-cleavable linker, and a potent microtubule inhibitor. Many current ADCs targeting solid tumors have higher doses (3.75-20mg/kg) than Kadcyla over a 3-week period, significantly exceeding many approved ADCs targeting hematoma (for example, Besponsa and Zynlonta have doses of approximately 0.02 and 0.15 mg/kg, respectively). These doses are necessary to overcome high expression and effective internalization to deliver the payload to the cells.
Other Design Criteria
Payload selection is critical to ADC development. Today, most payloads fall into one of three categories: (i) DNA damage inducers, (ii) microtubule inhibitors, and (iii) topoisomerase inhibitors. DNA damage payloads are usually very potent (e.g. PBD), while microtubule (DM4, MMAE) and topoisomerase (exatecan, SN-38) inhibitors are more modest. Determining the optimal payload requires a case-by-case analysis. Lower payloads provide greater maximum tolerated dose (MTD). However, for indications with low antigen presentation, payload delivery may not exceed the therapeutic threshold, and a higher potency payload is required. In addition to potency, recent studies have identified multiple payloads as inducers of immunogenic cell death (ICD). The ability of some payloads to induce an immune response after cell death is a new avenue of research that may have broad implications for the next generation of ADC payload selection.
The selection of linkers is also critical, as non-cleavable linkers are generally more stable in plasma. At present, linker chemistry has also made remarkable progress. For example, the Kadcyla non-cleavable linkers lost 18.4% of its payload in four days, while the Enhertu cleavable linkers lost 2.1% of its payload in 21 days. However, even if the linker can reduce the ADC payload loss in the loop, it faces a more difficult challenge after systemic absorption and degradation of the ADC itself. Since most ADC doses do not reach the tumor, the ADC will be metabolized elsewhere in the body, releasing the payload at an undesired location. Payload, linkers, and binding sites can all influence where non-specific release occurs in vivo and dose-limiting toxicity (DLT).
Current FDA-approved drugs exhibit significant variability in these design features, suggesting the need to personalize ADCs for their specific targets. However, the similarity of high-dose antibodies and highly expressed targets, combined with preclinical evidence, suggest that tissue penetration and tumor saturation are key components of efficacy in solid tumors. Therefore, we need to go beyond Cmax and AUC to consider tumor tissue penetration and tumor saturation to design next-generation ADCs.
Systemic pharmacokinetics, toxicity and efficacy are key metrics to be measured during drug development. Toxicity and pharmacokinetics can quickly screen out poor drug candidates at an early stage. In contrast, testing efficacy requires greater effort, and tissue penetration plays an important role in this process.
The permeability of tumor tissue that affects the efficacy cannot be considered in isolation, but toxicity is equally important. For example, at a clinical dose of 3.6mg/kg, 43.1% of patients treated with Kadcyla showed grade 3 or greater adverse events. Investigators are investigating alternative delivery methods using ADCs to potentially increase tolerability, such as batch dosing. However, when the efficacy of Kadcyla was reviewed for single versus batch administration, the clinical benefit of batch administration was reduced. Thus, a reduced dose and increased frequency of administration may be more tolerable, but smaller doses generally result in lower plasma concentrations, reduced tumor penetration, and lower targeting of tumor cells.
Another key aspect is tumor saturation. This depends on a variety of conditions, including dose (Cmax), expression (receptor/cell), internalization rate, and plasma clearance. The importance of tumor saturation is reflected in two ways: saturation dosage are more likely to be used in preclinical models; and saturation dosage may have opposite results to subsaturation dosage. First, the dosage administered to mice did not always correspond to clinically tolerated dosage. Dosages are sometimes increased in response to faster clearance or less responsive tumor models in mice, or because they are better tolerated in mice. This may lead to saturation in preclinical models, while clinically tolerated dosages may be subsaturated. Because mouse cells typically do not respond to ADC payloads as well as human cells and require high dosages, these high dosages may mask delivery problems in the clinic.
Second, the results of preclinical studies at saturation dosage may produce results opposite to those at subsaturation dosage in the clinic. For example, if a saturation dosage is given, increasing the DAR is more effective, while when a supersaturation dosage is given, lowering the DAR may be more effective. Typically, doses are limited by payload toxicity, so comparisons are made at constant payload doses. When the tumor is supersaturated, the cancer cells receive the largest number of antibodies, so more payloads per antibody will yield greater efficacy. For supersaturated doses, the opposite is true. In this case, the ADC cannot reach all cancer cells, and increasing the DAR (at a constant payload dose) will reduce the amount of antibodies delivered, thereby reducing the number of cells targeted and killed. Conversely, reducing DAR and/or increasing the total antibody dose under these conditions improves tissue permeability and overall efficacy. Therefore, the payload MTD should be associated with the saturation dosage of the antibody to achieve maximum tissue penetration and efficacy.
In addition, the three recently approved ADCs for solid tumors all use payloads that can have a bystander effect. The bystander effect allows the payload to diffuse out of the target cell and into adjacent cells after release. In theory, bystander payloads could also increase tissue penetration beyond what the antibodies themselves could achieve. This could explain the increased efficacy exhibited by Enhertu compared to Kadcyla in the NCI-N87 mouse model, despite similar cellular potency. However, increased tissue penetration at higher antibody doses would still improve efficacy even when using ADCs with bystander payloads. Although bystander payload improves distribution, direct antibody delivery is more efficient than bystander killing, which explains the greater effect of higher antibody doses even with bystander payload.
Target selection, linker stability, and payload toxicity have been major considerations in ADC drug design over the past few decades. However, many ADCs that looked promising in preclinical studies ended up failing in clinical trials due to toxicity and/or poor therapeutic windows.
By analyzing the common characteristics of recently FDA-approved ADC drugs, it is clear that tissue penetration and tumor saturation are equally critical for successful ADC design. Therefore, data on the most important parameters of efficacy, including tumor penetration and saturation, need to be given special attention in preclinical studies. In an ideal design scenario, at least two pieces of information are known: the tolerable payload dose in humans and the antibody dose required to saturate the tumor target. With this information, we can maximize efficacy at clinically tolerated doses by modifying the DAR to deliver the highest tolerated payload to all tumor cells.
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. Key metrics to expanding the pipeline of successful antibody-drug conjugates. Trends Pharmacol Sci.2021 Oct;42(10):803-812
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