Immunotoxin and Taxol synergy results from a decrease in shed mesothelin levels in the extracellular space of tumors

Abstract

Recombinant immunotoxins are chimeric proteins composed of the Fv portion of a tumor-specific antibody fused to a toxin. SS1P (CAT-5001) is an immunotoxin composed of an antimesothelin Fv fused to a 38-kDa portion of Pseudomonas exotoxin A. Immunotoxins have been shown to be active in lymphomas and leukemias, but are much less active against solid tumors. We recently reported that Taxol and other chemotherapeutic agents show striking synergistic antitumor activity in mice when immunotoxin SS1P, which targets the mesothelin antigen on solid tumors, is given with Taxol. Using a pair of Taxol-sensitive and Taxol-resistant KB tumors equally sensitive to immunotoxin SS1P, we examined the mechanism of synergy. We show that synergy is only observed with Taxol-sensitive tumors, ruling out an effect of Taxol on endothelial cells. We also show that the KB tumors have high levels of shed mesothelin in their extracellular space; these levels increase with tumor size and, after Taxol treatment, dramatically fall in the drug-sensitive but not the drug-resistant tumors. Because the mesothelin levels in the tumor exceed the levels of SS1P in the tumor, and because shed mesothelin is being continuously released into the circulation at a high rate, we propose that synergy is due to the Taxol-induced fall in shed antigen levels.chemotherapy ͉ drug resistance ͉ immunotherapy ͉ mesothelioma ͉ ovarian cancer A ntibody-based therapies now play an important role in cancer treatment. These therapies include humanized or fully human unmodified antibodies, as well as antibodies used to deliver toxic payloads such as radioisotopes, toxins, and lowmolecular-weight cytotoxic drugs (1, 2). One well recognized impediment to effective antibody-based therapy is tumor penetration. Several factors contribute to this poor penetration, including a defective vasculature characterized by abnormal blood vessels, a lack of functional lymphatics so that proteins can only enter slowly by diffusion, and elevated interstitial fluid pressure generating an outward flow of fluid. There also is a site barrier limiting the entry of antibodies and other antibody-based therapeutics caused by the binding of antibodies to the first layer of cells they encounter as they leave capillaries within the tumors (3, 4). Another factor is a collagen-rich and stiff extracellular matrix (5). Some of these barriers also can affect the entry of standard low-molecular-weight chemotherapeutic agents (6). One way to improve the uptake of antibodies by tumors is to administer large amounts, achieving high blood levels. However, this strategy cannot be used with immunoconjugates and immunotoxins because the cytotoxic payload produces side effects not observed with naked antibodies (7,8). To improve the efficacy of antibody-based therapies, they have been combined with chemotherapy, and striking increases in antitumor activity have been observed (9-12).Recombinant immunotoxins are genetically engineered proteins composed of the Fv portion of an antibody fused to a bacterial or plant t...

ImmunoPET with Anti-Mesothelin Antibody in Patients with Pancreatic and Ovarian Cancer before Anti-Mesothelin Antibody–Drug Conjugate Treatment

“…Hepatic catabolism might be promoted by the antibody complexing with MSLN antigen shed into the circulation. Shedding of antigen into the tumor interstitium is a well-known process for cell-surface proteins including MSLN, which can also influence tumor uptake of MSLN targeting agents in preclinical models (41,42). However, liver uptake levels of this antibody are comparable with that of other antibodies, such as trastuzumab and huJ591 (26,43).…”

High Shed Antigen Levels within Tumors: An Additional Barrier to Immunoconjugate Therapy

“…Using a method developed by Wiig et al (50) to measure albumin present in the extracellular fluid (ECF) of tumors, the levels of shed mesothelin in the ECF of tumors was measured and found to be extremely high, up to 100 nmol/L, greatly exceeding the amount of immunotoxin in the tumor (up to 10 nmol/L). In addition, soluble mesothelin was being produced and released into the blood very rapidly (47). Recently, investigations in our laboratory showed high levels of two other shed antigens in tumors: CD22 in the ECF of CA46 lymphomas and transferrin receptor in KB tumors.…”

“…The levels of mesothelin within the ECF of KB tumors were measured before and after Taxol treatment, and were found to decrease dramatically over the 2-to 5-day period after treatment as tumor cells underwent apoptosis (47). Presumably, this decrease in shed antigen is due to the slowing of synthesis by dying and dead tumor cells and the increased transfer of antigen from the ECF into the blood.…”

“…In all cases, synergy was observed indicating the response is quite general. Only when the tumor was drug resistant was synergy not observed (47). Because it is known that entry of antibodies and immunoconjugates into solid tumors is poor (2), the effect of chemotherapy on immunotoxin uptake by the tumors was measured, and no increase in total immunotoxin uptake was detected, indicating another mechanism was needed to explain synergy (46).…”

Single-Chain Antibody-Based Immunotoxins Targeting Her2/neu: Design Optimization and Impact of Affinity on Antitumor Efficacy and Off-Target Toxicity

“…More importantly, the current study clearly described the increased potential for high-affinity immunotoxins to form immune complexes in vivo resulting in off-target toxicity in liver responsible for clearing these complexes. These data have potential relevance to a number of anti-Her2/neu approaches (23,24), as well as other targets for which there is a level of circulating antigen present (25,26).…”