A pair of scientists in EPA's research office is proposing a scaled-back version of the first round of testing currently required by the agency's Endocrine Disruptor Screening Program (EDSP), an effort that one of the scientists says is being welcomed by toxics office officials, who are struggling to operate the nearly 20-year-old program.
The two EPA research office scientists, Gerald Ankley and L. Earl Gray Jr., last month published a paper outlining their suggestions for how to more efficiently and affordably screen of chemicals suspected of disrupting the human endocrine, or hormone, system.
Rather than requiring chemical manufacturers to run the chemicals through all 11 screening assays that EPA currently requires for Tier 1 EDSP testing, the scientists are instead suggesting that chemicals be screened through only two of the assays to determine if additional screening is required.
Gray presented the proposal, which he calls a "logic-based decision tree" at an April 23-24 workshop on the EDSP program in Research Triangle Park, NC, which was run by the independent risk assessment consulting group Toxicology Excellence for Risk Assessment and sponsored by industry and animal rights groups.
In his presentation, Gray called the two screening assays the "gatekeeper" assays, and suggested that if these two assays indicate there is not endocrine activity, the chemical should be placed on hold status, and the remaining Tier 1 assays should not be executed.
But if the gatekeeper assays suggest estrogen, androgen or thyroid activity, additional assays would be performed based on the effects seen in the gatekeeper and any subsequent assays, Gray said.
He said that the decision tree is "not complete, and still a work in progress," and added that he and Ankley "presented this to the program office, who were wildly enthusiastic."
The paper, "Cross-Species Conservation of Endocrine Pathways: A Critical Analysis of Tier 1 Fish and Rat Screening Assays with 12 Model Chemicals," first appeared on the website of the journal Environmental Toxicology and Chemistry March 19.
When Congress first mandated the EDSP program in 1996, it was intended to provide a quick and cheap screen of substances' potential to interfere with the human endocrine system.
Instead, the two-tiered program's first tier screen ballooned into 11 in vivo and in vitro assays, with a total cost of as much as $1 million, speakers indicated during the recent workshop.
Over the course of the past year, companies provided results from the Tier 1 assays that EPA required for some 67 pesticides that were on the first list of chemicals to undergo Tier 1 screening, and agency staff is now reviewing the results of those assays.
But many industry and animal rights groups are concerned about the cost of complying with EPA's screening mandates.
To address this, Gray and Ankley propose running only two in vivo "gatekeeper" assays -- the fathead minnow and pubertal male rat assays included in EDSP Tier 1, Gray said. Positive results in these assays could trigger further testing through other Tier 1 assays, such as the Hershberger, amphibian, pubertal female rat or aromatase assays.
Gray explained that in vitro assays cannot be gatekeeper assays because they cannot account for absorption, distribution, metabolism and excretion [ADME] and are also missing pathways of estrogen, androgen and thyroid (EAT) and cannot predict "other modes of toxicity."
While the agency is seeking to gradually transition toward various cellular or computational efforts to perform future endocrine screening, Gray raised questions about high-throughput cellular testing, saying that these "assays are even more limited [than the in vitro assays in the existing Tier 1 screen] at this point in time" and have not been validated by the Organization for Economic Cooperation & Development.
Gray also proposed an interim step before screened chemicals went on to EDSP Tier 2 testing, which EPA has yet to finalize. While the agency has indicated for years that a half-dozen assays will make up the second tier, several remain to be validated, as required by law for inclusion in EDSP.
Gray called his interim step Tier 1.5, and describes it on his presentation slides as "additional in vitro or short-term in vivo assays to confirm equivocal tier one screening results or to explore potential EAT effects in more detail before initiating extensive tier two testing."
"If we run through tier 1 and it's equivocal, does that mean our only option is to run Tier 2 [assays]?" Gray asked rhetorically. "That doesn't make sense."
Others at the conference proposed additional steps that EPA might consider as it moves forward with the long controversial program. Rick Becker, senior toxicologist with the American Chemistry Council (ACC), pressed the agency to place the program on hold while staff review the myriad results companies provided in the results of screening the first list of chemicals and consider their meaning.
"Maybe it's time for a hard stop, sit up and look at what we've got and at the same time, accelerate ToxCast," Becker said. "I'm concerned we could do all the testing and what have we gained? Are we ready for a hard stop, for a year or two?"
Becker also suggested that EPA create a new advisory committee for the EDSP program, similar to the existing Pesticide Program Dialogue Committee (PPDC), a group of stakeholders who provide input to the agency's pesticides program. "Seems to me, there is some potential for a PPDC," Becker said during a discussion at the end of the workshop. "It seems to me there's a lack of a group like that with EDSP an yet there's a greater need for that more than ever."
The meeting's content and discussions will be included in a published report. Upon completion, the report will be provided to EPA, meeting organizers said.
Separately, the Endocrine Policy Forum, an industry group representing companies that received test orders for EDSP List 1 chemicals, is raising concerns over the testing assays, which they say need more refinement before being used to prioritize chemicals for testing in order to reduce the number of false positives produced that will necessarily subject a chemical to more expensive Tier 2 testing.
"We want to know that Tier 1 screens are going to be relevant for sending us to Tier 2 tests, relevant for for risk assessment," Ellen Mihaich, scientific coordinator for the Endocrine Policy Forum, and head of the consulting firm Environmental and Regulatory Resources, LLC, told a session of the CropLife America spring meeting April 19 in Arlington, VA. She added that industry still supports an eventual move to high-throughput methods available under EPA's ToxCast program, but only if it's done right.
Under the ToxCast program, which is part of the government-wide Tox21 Initiative, EPA is developing ways to conduct in vitro and in silica methods for toxicity testing in an effort to replace more costly and time-intensive animal testing. Though EPA has had considerable advances in the ability of the program, and has screened thousands of chemicals, it is yet to be considered sufficient to use in a regulatory context.
"We aren't trying to bad mouth [ToxCast]," Mihaich told Inside EPA in an interview on the sidelines of the conference. "What we are saying is more work needs to happen . . . if we are going to use it to replace studies, we need to be confident" in the results.
Mihaich told the meeting that given that the battery of testing required under EDSP Tier 1 for each chemical is $750,000 to $1 million, "it is not trivial and we want to get it right."
In her presentation, Mihaich said that when data from List 1 chemicals from the current Tier 1 screening battery, which includes both animal and high-throughput screening assays was compared with data run on those chemicals entirely through ToxCast, the ToxCast data resulted in a far greater incidence of false positives, which in some cases were as high as 50 percent.
While the group is supportive of moving to high-throughput screening for EDSP, the "methods, assays and data need to be appropriately validated," Mihaich said.
EDSP "is a work in progress for both EPA and for us," she continued, adding that while the group was not able to look at all the data for the Tier 1 chemicals to compare false positives, the review "gives us an idea of what's going on." -- Maria Hegstad & Jenny Hopkinson