For the current week, the overall ratings of eight Biotechnology stocks are worse, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

This week, Merrimack Pharmaceuticals Inc. (NASDAQ:MACK) falls to a D (“sell”), worse than last week’s grade of C (“hold”). Merrimack Pharmaceuticals, Inc. is a biotechnology company, which is focused on the discovery, development and commercialization of drugs for the treatment of immunological and autoimmune diseases with an initial focus on cancer. The firm’s product candidates include MM-398, which is a novel, stable… For Portfolio Grader’s specific subcategory of Cash Flow, MACK also gets an F. For a full analysis of MACK stock, visit Portfolio Grader.

This week, Genomic Health (NASDAQ:GHDX) drops from a C to a D rating. Genomic Health is a life science company, which is focused on the development and commercialization of genomic-based clinical diagnostic tests for cancer that allow physicians and patients to make individualized treatment decisions. The stock also gets an F in Earnings Revisions. The stock has a trailing PE Ratio of 116.00. For more information, get Portfolio Grader’s complete analysis of GHDX stock.

Acorda Therapeutics (NASDAQ:ACOR) is having a tough week. The company’s rating falls from a C to a D. Acorda Therapeutics is a commercial stage biopharmaceutical company dedicated to the identification, development and commercialization of novel therapies that improve neurological function in people with multiple sclerosis (MS), spinal cord injury and other disorders of the central nervous system. The stock gets F’s in Earnings Revisions and Sales Growth. To get an in-depth look at ACOR, get Portfolio Grader’s complete analysis of ACOR stock.

This is a rough week for Amarin (NASDAQ:AMRN). The company’s rating falls to D from the previous week’s C. Amarin focuses on developing the treatment for cardiovascular disease in the field of lipid science. In Earnings Growth, Earnings Revisions, Equity, and Cash Flow the stock gets F’s. The stock price has dropped 9% over the past month, worse than the 0.8% increase the Nasdaq has seen over the same period of time. As of April 17, 2013, 18% of outstanding Amarin shares were held short. For a full analysis of AMRN stock, visit Portfolio Grader.

Exelixis (NASDAQ:EXEL) ratings are on the decline this week as the company earns an F (“strong sell”). Last week, it received a D (“sell”). Exelixisis a development-stage biotechnology company dedicated to the discovery and development of small-molecule therapeutics for the treatment of cancer and other serious diseases. The stock receives F’s in Earnings Growth, Earnings Momentum, and Equity. Margin Growth and Sales Growth also get F’s. As of April 17, 2013, 20.4% of outstanding Exelixis shares were held short. For more information, get Portfolio Grader’s complete analysis of EXEL stock.

Trius Therapeutics (NASDAQ:TSRX) earns a D this week, falling from last week’s grade of C. Trius Therapeutics is a biopharmaceutical company. The stock gets F’s in Earnings Growth, Earnings Momentum, and Equity. Cash Flow and Sales Growth also get F’s. Wall Street appears to agree with the stock downgrade, with share prices dropping 6.2% over the past month. To get an in-depth look at TSRX, get Portfolio Grader’s complete analysis of TSRX stock.

Slipping from a C to a D rating, Discovery Laboratories (NASDAQ:DSCO) takes a hit this week. Discovery Laboratories is a biotechnology company focused on developing products for the treatment of respiratory disease. The stock gets F’s in Equity and Cash Flow. The stock price has fallen 21.6% over the past month. For a full analysis of DSCO stock, visit Portfolio Grader.

Achillion’s (NASDAQ:ACHN) rating weakens this week, dropping to a D versus last week’s C. Achillion focuses on the discovery, development and commercialization of innovative treatments for infectious diseases. The stock also rates an F in Equity. Share prices fell 17.8% over the past month. As of April 17, 2013, 13% of outstanding Achillion shares were held short. To get an in-depth look at ACHN, get Portfolio Grader’s complete analysis of ACHN stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.