Heptamethine dyes

Optical properties

The absorption and fluorescence spectrum of heptamethine dyes are in the near infrared region and vary largely on the solvent and concentration. IR-808 absorbs mainly between 775 nm and 796 nm and emits fluorescence between 808 nm and 827 nm with a large red shift as expected in the serum sample. These absorption rates are important for their medical applications due to the image capturing capabilities. Dyes with good pharmacokinetics and fluorescence above 680 nm are considered viable in biomedical imaging due to their significant tissue penetration and imaging signature relative to background frequencies. IR-808 and other analogs produced well defined images with excellent visualization of cancer tumor cells.

Toxicity and side-effects

Heptamethine dyes are metabolized in the liver and excreted via the bile ducts. In experiments this was exhibited by viewing extensive fluorescence only in the fecal matter of mice without tumors in the removed intestines after the dye had time to propagate in the test subject’s body. This implies that the dye is effectively removed from the systemic system by the liver and cleared from major organs into the intestinal tract via the bile duct within 48 hours. Further tests in humans and imaging exposure to NIR light have yet to be performed. When the mice were injected with 500 times the necessary dose there appeared to be no side effects noticed which indicates that this dye contains excellent possibilities as a biomedical imaging tool.

Diagnostics

The in vivo accumulation of dyes in tumor cells are being explored using HeLa cells in mice to explore the possible use as a tool in the growing field of theranostics. These tools show a promising ability to increase the survival rate of individuals undergoing cancer treatment by using a personally designed treatment method based on the increasing amount of patient specific diagnostic data available. In heptamethine dyes the contrast index, which measures the relative emission of the dye fluorescence in tumor cells relative to non tumor cells, was found to be considerable if the value observed exceeded 2.5. In the IR-808 complex the CI exhibits 25 at 10 days after the HeLa tumor producing mice are injected with the dye which indicates this specific complex exceeds the criteria as an effective diagnostic agent. Upon further investigation with rhodamine 123 the dye was found to target specifically the mitochondria of the cells. This indicates the dye targets the portion of the tumor cell which is vital to cellular survival.

While the IR-808 is attached to the cancer cells the target organism can be imaged with NIR fluorescent imaging techniques. When overlaid with the x-ray of the organism the exact location and extent of the tumor cells can be determined (figure 2). [1] Being able to detect small tumors with great accuracy and no side effects is a groundbreaking and very powerful tool in early diagnosis of cancer tumors. It has been shown that physicians using this type of dye improve the accuracy of detecting cancer cells to around 96%. [2]

Perfusion diagnostics of tissues and organs

IR-808 and other analogs have been shown to perform as a marker in the assessment of the perfusion of tissues and organs in HeLa tumor growing mice. The light used to excite the tumor bound dye is a near infrared light. The fluorescent signature can then captured by a recording device such as the Kodak in-vivo Professional Imaging System. Being able to capture the image means that the extent and location of cancerous cells in capillary beds throughout the body can be assessed and documented. Therefore, the possible uses in medicine of just the imaging effects of heptamethine dyes would allow the earliest detection and further monitoring of tumor cell growth in one of the smallest transport systems of the body without the use of radioactive chemicals in a very short time. The capabilities of heptamethine dyes are also novel in that they recently have been found to show the capabilities of killing tumor cells when co-administered with a PDT.

Uses in PDT - Photodynamic Therapy

The heptamethine dye IR-808 is unique in that it has been found to not only attach to cancer cells independent of other mechanisms, but it has also been found to exhibit cytotoxic properties when irradiated within 6 hours of administration of the dye. The tumor cells selectively take up the dye as previously described and then are exposed to light wavelengths at the same frequency, in this case 808 nm, as the dye absorbs. In mice the cancer tumor cells of the LLC cell line with the dye and irradiation were found to have significantly decreased in both mass and volume. HeLa cell viability in vitro appeared to have the lease amount of cytotoxicity when compared to other cell lines and similar Photodynamic Therapy methods. Further research into the efficacy and methods of varying cell lines would verify the validity of its use as a legitimate anti-tumor agent independent of other tumor suppressing drugs. Nevertheless, the opportunities of a biomedically safe and minimally invasive cancer targeting, imaging, diagnosing, and tumor reducing agent in IR-808 and possibly other heptamethine dyes remain an exciting and promising field worth further research.

Patents

International patents on the use of heptamethine dyes for use in photographic use have also been approved and instituted. Fuji film placed a patent titled “Heptamethine cyanine compound, near infrared absorbing ink, near infrared absorbing sheet and silver halide photographic material” in 1999 and a follow-up international patent 6072059 in 2000. [3]

Heptamethine dyes for near infrared fluorescent imaging are manufactured by such companies as Intrace Medical, Lumiprobe, Cyandye, LI-COR Biosciences - GmbH and others.