conversion of the drug from a nontoxic form to a tumor-killing form. This conversion is
achieved uniquely within the tumor cells by shining the activating light only upon the
tumor. Elsewhere within the body the drug remains in an inactive, nontoxic form.
Porfimer sodium is an agent used in photodynamic therapy. Porfimer is an oligomeric
mixture of porphyrin units linked by ester and ether linkages. The porfimer selectively
accumulates in tumor tissues and is cleared from other tissues. Laser illumination of the
tumor at 630 nm wavelength then causes the porfimer molecule to enter an excited state
in which singlet oxygen is produced by radical reactions. As this reaction propagates
(by spin transfer from porfimer to molecular oxygen), hydroxyl radicals are produced.
These are locally toxic to the tumor.
7.8.6 The Clinical–Molecular Interface: The Cell Nucleus
and Cancer TreatmentCancer is the common term for any of a group of diseases characterized by abnormal
cellular growth that typically produces malignant tumors.
Typically, a tumor is regarded as the pathological hallmark of cancer. The term
tumor, however, is a nonspecific term for any swelling or localized enlargement.
Tumors that may potentially be cancerous are more correctly categorized as neoplasms.
A neoplasm is an abnormal mass of tissue whose growth is uncoordinated and exceeds
that of normal tissues and whose presence persists in an excessive manner following the
termination of the stimulus that provoked the initial formation of the abnormal tissue
mass. A neoplasm may be either benign or malignant (“cancerous”). A neoplasm is a
benign tumour when it is a slow-growing, localized, expansible mass, enclosed within
a capsule; benign tumours have well-formed and differentiated cellular elements
(i.e., subcellular structures, such as organelles and the cell nucleus, appear morphologically
normal and resemble those of cells from normal tissues). A neoplasm is a malignant tumor
when it is a fast-growing, non-encapsulated, infiltrative, erosive growth of cells capable
of being transported from its site of origin and being implanted in other sites (with this
cellular relocation process being referred to as metastasis); malignant tumors have poorly
formed and poorly differentiated cellular elements (i.e., subcellular structures have
altered morphology and do not resemble the typical nucleus and organelles of normal
cells). Malignant tumor cells may be described as anaplastic when their subcellular
structure is more or less undifferentiated and when the cells have lost all resemblance to
normal counterpart cells. Anaplasia of cellular morphology is an unmistakable hallmark
ENDOGENOUS CELLULAR STRUCTURES 459