Introduction and Background of the Project

Introduction

Soil-transmitted helminth (STH) infections are a significant neglected tropical disease affecting approximately 1.5 billion people worldwide, or 24% of the world’s population. The different species of intestinal parasitic worms (hookworm, roundworm, and whipworm) infecting humans are transmitted through contaminated soil, placing the heaviest burden on rural, low socioeconomic status communities lacking access to clean water and sanitation. Infected infants and children are at highest risk of mortality and morbidity from STH infections. The insidious effects on health and quality of life resulting from STH infections include low birth weight, iron deficiency anemia, chronic malnutrition, stunting, impaired growth and physical development, cognitive development disorders, delayed educational advancement, and a negative impact on economic development. In 2018, over 676 million schoolchildren were treated with anthelminthic medicines in endemic countries, but this only accounts for 53% of all children at risk.

Project objective

This project seeks to further the development of a new first-in-class broad-spectrum anthelmintic drug product that can serve as an inexpensive and potent alternative or complement to benzimidazole (e.g., albendazole and mebendazole) treatments currently used to combat STH infections and associated disease burden. Building on the work from the previous grant, the objective of this project is to optimize the Bacillus thuringiensis crystal (Cry) protein Cry5B lead sequence for Trichuris, commonly known as whipworm, which is the most difficult target because of its feeding habits and its location in the large intestine. Trichuris trichiura is a parasitic nematode that infects humans to cause trichuriasis, also known as human whipworm infection. Whipworms are fully sensitive to Cry5B and Cry5B variants tested in vitro. Based on previous studies, naturally occurring amino acid substitutions may significantly increase the bioactivity and effectiveness of Cry5B protein variants against hookworms. Variant screening will improve two parameters of Cry5B protein activity: effective dose and time to kill. These studies will result in the identification of a Cry5B variant lead candidate that will bring the whipworm effective dose down to the hookworm dose of 1 mg/kg.

Project design

In exploratory studies conducted under the previous grant, the University of Massachusetts Medical School (UMMS) team began testing the activities of several Cry proteins and natural Cry5B amino acid variants against Trichuris. The sequence differences between Cry5B and variant Cry5B and other Trichuris-active toxins are hypothesized to affect toxin stability, activation, or receptor avidity. Under this project, UMMS will systematically test Cry variants for improved activity against Trichuris and, secondarily, hookworm (roundworm activity tracks with hookworm activity). Identified improved Cry5B amino acid variants and new Cry proteins will be produced by the Kao Corporation as Inactivated Bacterium with Cytosolic Crystal (IBaCC) in their Bacillus subtilis-based expression system, and then tested in vitro and in vivo at UMMS. The robust Bacillus subtilis expression technology for producing large amounts of recombinant Cry5B anthelminthic protein achieves a level expression of several grams per liter of culture, with the potential for further improvement. Our technology will produce and purify Cry5B at the scale necessary for biochemical characterization, purification, and subsequent in vitro and in vivo bioactivity testing to select the lead potency Cry5B protein variant candidates for advancement. As protein variants superior to canonical Cry5B are identified, they will be tested in vivo against whipworm infections in mice.

How can your partnership (project) address global health challenges?

No new drugs to treat STH infections have been introduced in more than 30 years. The drugs most widely used in mass drug administration campaigns against STH (the benzimidazoles albendazole and mebendazole) are threatened by the rise of resistance, which has already rendered some nicotinic agents, such as levamisole, largely ineffective in the treatment of livestock with parasitic nematodes. A significant gap in the profile of all of the currently approved anthelmintic drugs is that they are contraindicated for women in their first trimester of pregnancy, which leaves the mother and her fetus vulnerable to the effects of STH-induced anemia and malnutrition, and out of reach of a therapeutic intervention. As a non-absorbed therapeutic agent that binds to a target known only in invertebrates, Cry5B is expected to have an excellent safety profile for all patients, including pregnant women and young children. Cry5B would be an inexpensive and potent addition to the therapeutic options for managing of STH infections and can form part of a combination therapy with existing drugs. It acts synergistically with some nicotinic agents, and resistance to these can make STH hypersensitive to Cry5B.

What sort of innovation are you bringing in your project?

Cry5B is a powerful, promising new candidate for a completely novel anthelmintic drug product. It has a unique mechanism of action to treat all major STH infections and is unaffected by resistance to existing drugs. The ultimate goal of this project is to develop a safer and more effective alternative to existing treatments that is cost-effective, massively scalable, acceptable for use in low resource settings (e.g., in mass drug administration), and safe to use in all at-risk populations, including children and pregnant women.

Role and Responsibility of Each Partner

Kao Corporation is a major Japanese consumer products and chemicals manufacturer. Kao's mission is to strive for the wholehearted satisfaction and enrichment of the lives of people globally and to contribute to the sustainability of society. Kao is a leader in the use of Bacillus in their production of industrial enzymes and will be responsible for the production of recombinant Cry5B proteins. PATH is the leader in global health innovation. PATH brings our entrepreneurial insight, scientific and public health expertise, and passion for health equity to this project, as well as over 40 years of experience in working with partners on global health issues. PATH’s role is to facilitate activities at Kao and the UMMS and provide support in designing studies for physicochemical evaluation (such as solubility, stability, aggregation) of lead Cry5B variants, which will be critical for circumventing the challenges with stomach acid and enzymatic degradation and for developing appropriate formulations for oral delivery of Cry5B in the next phase of the project. Dr. Raffi Aroian of the UMMS is a leading expert in STHs. Dr. Aroian and his laboratory at UMMS was the first to characterize Cry5B and its mechanism of action. From the beginning, Dr. Aroian has championed the potential of Cry5B as a new anthelmintic that could have a significant impact on the management of the global STH problem. Dr. Aroian’s laboratory has established the animal models required to test the efficacy of Cry5B and combinations thereof against a number of parasitic nematodes. UMMS has made the first attempts at expressing Cry5B in other bacteria and has all the necessary reagents and tools to work with Cry5B. UMMS will design the Cry5B variants and provide Kao with technical guidance regarding strain development and purification. UMMS will also be responsible for in vitro and in vivo testing.

Final Report

1. Project objective

The overall goal of this project was to advance the development of a novel crystal (Cry) protein as a first-in-class broad-spectrum oral anthelmintic drug product for treatment against soil-transmitted helminth (STH) infections, with emphasis for pregnant women and children. Although active against hookworms and roundworms in the small intestine, sequence optimization for efficacy was required to improve activity since in vitro activity was 10–100X less efficient against whipworms in the large intestine. Therefore, our project objective was to optimize the lead sequence for improved potency against Trichuris (whipworm).

2. Project design

We designed and synthesized more than 500 sequence variants of Cry5B and Cry21A, and produced Inactivated Bacterium with Cytosolic Crystal (IBaCC) lysate using Bacillus subtilis–based expression technologies of Kao Corporation. Variants were tested at University of Massachusetts Chan Medical School for improved activity against whipworms and hookworms in vitro. The project team regularly met to review the results from the productions and preparations, rank the lead candidates, and select single amino acid mutants for combination into double and triple variants. Some of the best variants were scaled up and taken into in vivo mice studies against whipworms.

3. Results, lessons learned

We demonstrated that our approach to sequence optimization achieved significant improvements in anthelmintic activity over wild-type Cry protein. We confirmed the superior in vitro potency of at least one double Cry5B variant translated into in vivo efficacy improvements. We successfully detected that Cry21A achieved 70% reduction in whipworm fecal egg counts and a double Cry5B variant showed statistically significant 86% reduction in worm burden, both of which warrant further optimization. These promising results suggest that pursuing the combination and optimization of triple and quadruple variants of Cry5B and Cry21A will move towards the desired efficacy of 10 mg/kg against whipworms.

There were two important lessons and outstanding challenges that we have learned for optimizing Cry protein for large intestinal delivery and efficacy against whipworms that were not able to be addressed through sequence optimization alone: (1) less than 5% of the Cry protein dose survived stomach degradation, and (2) increased solublilty increases efficacy against whipworms. Further optimization of solubilization is called for. Each of the unique variants requires identification of an optimal buffer composition to improve protein stability, solubility, and accessibility to intoxicate whipworms. Our studies also suggest that optimizing the strain and fermentation characteristics of IBaCC for the lead sequence candidates concurrent with formulation can result in even greater improvements in its solubility and bioactivity.

The results of these experiments give us confidence that taking our proposed three-pronged strategy will lead to the identification of optimal variant candidates to meet the target product profile. Complementary research efforts encompassing (1) Cry5B and Cry21A combination sequence variants, (2) improved strain and fermentation process, and (3) novel formulation approaches leveraging a unique encapsulation system already developed by PATH is needed to accomplish the goal of producing an active and soluble form of Cry protein that can survive stomach passage and be immediately available for hookworm and whipworm ingestion in the small and large intestine, respectively. We have accomplished a method for improving the lead sequence for activity against both parasites; however, optimal protein structure, form, and formulation will be necessary to achieve an effective dose for a lead preclinical drug formulation candidate